I used to breakdance very poorly in college. There was (and hopefully still is) a community of poppers, lockers, and breakdancers that would meet every Tuesday night at a local pizza joint called Joe Squared to dance and show off their moves. Only a few of my friends did this. The rest of them were interested in other things: dungeons and dragons, games, indie music, art, and whatever else. The communities that formed around these interests typically didn't intersect. That's something most people can relate to: if you have multiple interests, you tend to build not just a social circle but multiple circles, frequently with little overlap. You wear a certain identity for each one. But when they come together in unexpected ways, it's sort of weird.
Fitting into communities gives you many benefits. Access to friendship, companionship, career opportunities, creative fulfillment, money, and more, depending on the nature of that community. It pays off to fit in. Before I would go dancing, I dressed urban, pumped myself up with music, and got in the mood to hang out with everyone. If I'm going to an art gallery, I'm probably going to dress in black and Google the artists being shown to at least have a passing chance at appearing knowledgeable. If I go to a tech meetup, I will make sure to wear one of my nicer T-shirts. We participate in communities and signaling our membership by adopting their uniform, behavior, and lingo.
These communities have a cultural symbology that indicates that you're "in" - and if you're not using that symbology, you're probably out. It's important to be "in" if you want to participate in that community. Social fragmentation is a natural and beneficial act that enables us to easily identify friends and kindred spirits. Unfortunately, large, centralized social platforms like Facebook and Twitter perform a defragmentation of the social identity, making it more difficult to signal your membership to a certain community without alienating the members of others.
Offline, it's relatively easy to achieve an almost total fragmentation: you can control where you are and who you are around, and it's unlikely for your friends from one community to suddenly appear in the environment of another community. Still, most of us have to play the online social game. The ubiquity of Facebook and Googling people has made not creating and curating a personal online social identity a poor choice. The centralization of over a billion people and probably almost all of your friends means that you can use Facebook and similar social sites to let your friends know what's happening (and broadcast your social value) all at once. In the information age, attention is a form of power, and Facebook and Twitter make it exceedingly easy to harness that power.
The issue is that now sharing online with content meant for one social circle can alienate members of another. This is more complex than the simple your-grandma-saw-you-swearing-on-Facebook trope (although that's an example of this too). This could be your pissing off a Republican friend from your cycling club with a shared Democrat post, a friend who is a very publicly committed BDSM enthusiast posting on your wall and upsetting your fairly devout mother-in-law, or even simply making others who don't do something you do feel an emotional distance because there's no way for them to interact with you about the thing that you do. You can see the truth of this play out in the fact that your most popular posts will typically be mundane yet relatable (I sneezed on my food), unequivocally positive (I won an award!), or universally unoffensive (picture of a kitten). My most recent popular Facebook post was about my buying an iron.
There are solutions to this problem, which all have their own unique costs. You can use privacy settings to hide certain posts on Facebook, but it takes a lot of effort to exclude specific people, groups, or only include certain people. Even then, there's no guarantee that just those people will see it, and you might have a suspicion that someone else outside of the group might appreciate the post (but you don't know who that would be).
You can create an "alt" account specific to certain interests, but then you incur the cost of inconvenience in terms of managing, logging into, and out of various accounts. You also incur concrete social costs: if it's very important to keep your alt account anonymous, you can't use your "social content" in more than one place. You risk linking accounts when you mention that you got a new puppy both on your main Facebook and on your anonymous porn Tumblr (and boy, do you want to share a picture of that puppy, too). Friendships on alt accounts also tend "graduate" to your real account once they grow strong enough because you want those people to learn more about you so that you can have a more fulfilling, complete relationship with them.
It gets more complex. It becomes increasingly easy to become socially "bubbled" on Facebook. Its algorithms influence who sees your posts. The people who interact with you the most see your posts the most, and further increase this tendency when they like or comment on them. The algorithm bubbles them to you. Whether you know this or not, you know that they're likely to interact with your posts the most because they have in the past and continue to do so. I think this makes it easy to feel an unconscious (or maybe conscious) desire to appeal to those friends in order to strengthen the relationship and be a part of that community. It's incredibly easy to strengthen the bubble through this feedback loop, which can lead to an unconscious optimization for the most social reward, thoroughly locking your identity.
This isn't necessarily a bad thing. I'm not claiming that Facebook's algorithm is bad for us. If anything, the way it (seems) to behave is probably how I would want it to be implemented. We make friends and we bring them closer. But I do think that it's important that the implications of what Facebook's algorithm--and any other existing or future centralized social platform--means for our social reality. Even before the internet, we still had the constraints of time and energy when it came to juggling our personas around friends, family, and communities. The fundamental problem has not changed, but social media brings it more nuance and reasons to carefully consider who to include and exclude from your world.