Have an account for a year or so now. Kind of off of it for a few days as I'm going through that 'forgotten' feeling you get when friends are posting about all the awesome stuff they're doing and you're not a part of it, but rather facing a lonely apartment and nothing to do on the weekend.
Granted, these friends are either ones who have moved off to other cities/countries or have their own families to do stuff with or there's a noticeable age-difference and they have their own mainstay friends to hang with, but it still offers the same feeling, like being FB'ed-out.
I do sometimes get into a habit of posting 'negative' stuff, but try to catch it before I start a negative-string of posts.
You really want to reach out to people to let them all know what you're going through, but - as coyote said, "very few people have perfect, trouble-free lives. most of us have problems of one kind or another, most of us experience pain and unhappiness.", and sharing your portion of it more than a couple times, or even more than once, can get you a bad rep and maybe even a few unfriendings (or they just unsubscribe and it's like talking to the wind).
If you've ever watched postings by others who share their personal problems, it does come away as depressing and from someone who can't get their act together.
Which is ironic, since they're most likely asking for help in a non-direct manner.
It's not like you CAN'T post something about something bad in your life, but from my FB friends it's usually a once-every-few-months thing and it's always followed by one of their closer friends writing back that they'll give them a call and help them through it.
Something some of us can't expect from our posts.
FB is a happy-place. It's sites like THIS where we can let the demons out, even if our 'friends' on FB won't know about it.