Lately I've been thinking about how good Facebook is at what it does, and how it has become a unique cultural venue for people to write and be read and to stay in touch with casual acquaintances across gaps in time and space.
There have always been ways of staying in touch with people you were close to: e-mail and various instant message programs online, regular mail and phone, but those all had terrible "discoverability" (you had to get the address or number though some other channel) and were almost exclusively one-to-one communication.
Online, there have been one-to-many forms of communication: Usenet newsgroups and (God have mercy on your soul) website forums, but these were generally formed around mutual-interest topics and themes, not shared history in the real world.
Much of its strength comes from its ubiquity. Not being on Facebook is more of the exception than the rule.
Its curation algorithms are fantastic. I know some people balk at not seeing everything, but I don't think they realize what a firehose Facebook would become for anyone with a decent number of "friends". Facebook offers some tools to pay more attention to certain people you care about, but unlike some sites they don't force you to sort all your contacts into buckets, the tweaking is there if you need it. For everyone else, the algorithms do a pretty good job of bringing you the posts that other people have found most important. There's a bit of a bandwagon effect, and when you write a cool post that languishes uncommented and un-"liked" it's a bummer, but overall the system works well.
(Other people gripe about how oddly recalcitrant FB is about keeping feeds in strict chronological order... though I think the mix and match ordering based on time AND post activity works better for people who are more casually engaged.)
But Facebook banks on one brilliant idea, one other sites leverage as well: empowering users to assemble a collated page/wall/feed of content from people the user finds interesting. Sites using this trick -- Tumblr, LiveJournal, Twitter, Instagram and FB all had different hooks (visual collectors, diarists, pithy bon mot makers, snapshotters, and people you know, respectively) and of all of those FB's "people you know in real life" seems to be the most compelling in a universal kind of way. (Anecdotally, my high school's 20th, post-FB reunion didn't come together nearly as well as my 10th pre-FB, and while there were other factors involved I wonder how much of that is because the "where are they now?" question is so trivially answered.)
Facebook gets a huge number of UX and UI details so right. I do think the curation algorithms are under appreciated. There's no other site providing the non-geek with such a wide and known-IRL audience. Its photo handling is powerful and easy to use, and its instant messaging is a viable replacement for SMS/iMessage. Sometimes only being able to "Like" something feels limiting in a "Newspeak" kind of way, but it also cuts off a lot of negativity and fighting. Some previous annoyances (like endless game requests) have gone away for me. Other auxiliary features add to the experience: the "real time" event sidebar can lead to interesting discoveries (a kind of happenstance endure around the usual curation) and "what you posted on this date in previous years" is a good implementation of a nostalgic feature I've seen and implemented elsewhere.
My biggest complaint is about how this one site Facebook has sucked the air out of the room for the independent web and blogosphere. In the mid-2000s, my blog (which I still double post to, since it's my canonical archive) was also a small social hub, with a homebrew comments system that eventually got utterly deluged by robospam. (I also had a guest-post sidebar, http://kirk.is/sidebar/ that was great fun from 2002-2008) These days, only the most interesting and topical blogs can really survive and garner attention and community... Facebook has made things both more and less egalitarian in that regard.
There are other problems with Facebook, like how people put their own self-known private selves up against images of everyone else at their public best, and there's crap like vaguebooking, and privacy concerns with a machine that knows so much about mutual friends and even has face recognition. Or the idea that maybe the barrier to staying in touch should be high, like who wants to be in touch with those bozos from high school anyway, or have your elder folks know if you've been up to mischief, or see idiotic posts from that cousin whose politics you can't stand? But hearing and being heard is a very human desire, as is meaningfully staying in contact and having a support community of people you know, and FB does those things better than anything else I can think of.