Especially the liberals?
Unfriending has a lot of sources, but I find it perpetually strange that any people are surprised by the politics of others. Generally speaking, my engagement with politics has shown that... most people don't have any politics. They just sort of spout off without a coherent thought to why if they do bother to think about politics. This is especially true with younger people who often haven't settled on a political ideology leaning yet. So being "surprised" by a friend's sudden endorsement of the flavor of the month political figure or a random post that indicates some pro-life or pro-choice leanings shouldn't really be so surprising. It's basically random behavior that one should expect if they have enough social network "friends". Consider the finding in that polling data that only 37% of people say they post something political at all and you get the idea that most people don't care, think their political views if any should remain private, or don't have enough information to get into the debates that usually follow and so abstain from participating.
Usually this should be filed under "I think other people I know and like and respect are just like me, or should be as much like me as possible". And as is usually the case, most people are different. That's apparently where the surprise comes in. I find it baffling that people don't conceive of this beforehand. I suspect it is related to another social commentary I've noted of late: that most people wait to speak rather than listen. It makes for some very strange "conversations". But essentially the problem is that people, being human beings, have this ineffable interest in themselves and their own egotistical lives, and thereby expect others to be thoroughly interested as well. I find it very strange when they are interested in my life events and thoughts and therefore don't find it very strange that they should be much different and thus hold very different opinions and levels of knowledgeable support for said opinions than I should. Then again, I also hold some very odd opinions I'm sure.
The most interesting feature is the high frequency of liberals blocking and unfriending.
What I suspect explains the liberal tendency to unfriend (or block) over conservatives is one of two things however. I don't think it is explained by conservatives being apparently more tolerant (I suspect neither group ideology would fare very well at the extremes for tolerance, and neither has much of a claim to tolerance as a result).
1) Conservatives don't friend people who don't already share overt signals of cultural agreement as readily. They don't have much outreach to people of different and distinct religious or racial backgrounds in their immediate friends, so why should their digital friends be much different? If you don't have these disagreements as often, one would expect they wouldn't have much cause to unfriend. Liberals are more likely to have higher openness to experience and thus experiment with cross-ideological associations like these, and to give up on them with some frequency as well. They're also more likely to express things in cogent formats, with somewhat less obstinacy, or provide context when something potentially offensive goes up. That is a reflection on the relative dearth of modern conservative intelligentsia, but also a reflection of the more common folk nature available by sheer numbers. I've certainly encountered some very dogged and utterly stupid liberals in my time online too. But they're less frequent than the popular version I encounter of conservatives who think of me as a "socialist" for demanding more market-friendly schools or health care and calling for large reforms to entitlement spending. I find that line of argument much more offensive in its shear stupidity than the progressives who don't understand the impact of minimum wage or licensing laws on unemployment or prices or otherwise find these to be necessary interventions in the market for other suspect reasons.
2) "Conservatives" are more common than "liberals". (or at least, people who identify as strongly conservative are more common than strongly liberal, which would be the populations most likely to make significant numbers of politically charged postings, consider also that adding liberal, moderate, and conservative ideologies together gets you a figure of 204% of the internet population and 170% of the general. Lots of people are using more than one label, and traditionally speaking this is liberals). This means that liberals might come across the issue more often. Liberals are somewhat more common among the plugged in youth generation using social networks than among the general population, so this too may skew the sample.
As further possibilities, "moderate" is likely code for "doesn't care about politics", or at best, is likely able to grasp sometimes radically distinct politics by dint of having radically different (and sometimes conflicting) views themselves and thus doesn't care when other people have them either.
I find that most people will wish to use social networks the same way they use real friendships, with the idea of reinforcing and validating those ideas and concepts they already hold to, if any, and generally avoiding contact with hostile ideas, or ideas that they don't care about. Hence the similar percentage of people who they "friended" for agreement or whose posts they "liked" or expressed approval in comments for to the percentage of people who were unfriended or blocked, and the high percentage of people on the extreme ends who have friends of shared ideological views (or who think they have friends of shared ideological views).
They also, somewhat rightly in my view, use these networks to shun and deliberately exclude people who say more offensive and discriminatory things.
More than likely, many, many people have blocked me for the volume of political material I post. Possibly a couple have unfriended over it (at least one I should think, more likely I'm just tolerated and ignored). I will admit to having done so only once. A former classmate now living in California expressed a strongly religious backing while opposing a gay marriage law in California (Prop 8, being contested in federal and state courts now), and did so in a modestly prejudicial way causing some offence I'm sure. The religious views were not a surprise, so that didn't bother me. But after some reflection, I decided that I wasn't likely getting much out of other mundane posts being made by this person and wasn't all that interested in their comings and goings anyway and so removed them for the minor transgression of being offensive and the more significant transgression of being boring. I have other friends and some extended family members with modestly distinct religious views of their own, and plenty of mundane postings that I prefer to ignore most of the time. But none of them are likely to use the platform or their private religious views as a basis for a public post saying something prejudicial toward homosexuals. On occasion people post things with a view toward expressing some hostility to Republicans, though often this is for views and policies, or people expressing them, that I find reprehensible myself. I see the transparent "team" aspect of it, and sometimes I find that rather offensive in itself, but since conservatives aren't my team and is a team largely of affiliation by choice rather than affiliation through more deterministic means (biology), I'm a little less concerned. Presumably more conservative people concerned with politics post similar things on the opposite end of the spectrum. In fact, I'm quite certain they do.
That said, for my part, I also filter heavily my networking associations ahead of time and my networks remain smaller as a result (and presumably less interesting to google or facebook as a consequence). Most of the people I've removed have been more out of lack of interest or a lack of use (and in a couple cases, people dropped off the network entirely, removing themselves), rather than a lack of political agreement. I suspect this is the case for most of those 500 million unfriends. Politics certainly occupies a distinct portion of disagreement and disunion among friends and co-workers and families. I don't think it occupies the central core of those frictions.
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