I am not active on any form of social media. Unlike people of my age, I seldom open my Facebook account. I only do so, as my classmates often use this conveniently when uploading photos of field works and other pertinent files. Other than that, I rarely go online.
I once asked a friend the very same question. I told her that I often wonder why people chose to rant on Facebook/Twitter rather than talking to the person to whom the post is directed to. She told me that sometimes what she wants is just to let her emotions out and not gather attention from other people. She also isn't good at confrontations; hence, talking to the person whom she has issues with is very unlikely to happen. And this is where social media comes handy. Whenever she posts things about her misadventures on Twitter, it often comes in a series of tweets. She calls this act as "shouting to the void" because she doesn't really want other people to read it or comment on it. She's merely letting it all out.
I do understand her point of view but I guess this depends on one's preferences or as they say, "to each his own". I'd rather talk to my most trusted friend and ask for advice on certain troubles or just keep it to myself than have other people talk about what I've been going through.