FB is great so long as you are disciplined and use it sparingly. The danger of having every bloody person you met ever on there, and all your family, is that eventually you re going to hear/see something that pisses you off or makes you feel left out, or you will do that to someone else - sometimes unwittingly. The crossover effect is HUGE. Unless you are sending a PM it's always best to assume that anyone and everyone can and will see what you have just posted.
And if you are not discerning in who you add then you have to tolerate seeing/reading a lot of shit. I have my nieces and nephews on mine who are in their late teens, and I check up on my some of my own own DCs friends from time to time - teenagers are remarkably lax about their privacy settings. It seems to me that the scope for nastiness, tackiness and abuse over FB among younger people (or just among crass, stupid people) is almost limitless. 
I hardly ever do status updates. I have only about thee or four photos on there. I'd like to have more but I can't be bothered to learn how to do it. 
Mostly I just nose around at other people's more interesting (or not) lives, and share the odd funny photo with caption. I stay out of political rants, sob stories, 'life-affirming' schmaltzy nonsense, attention seeking cryptic updates designed to fish for compliments or sympathy, arguments, chain letters, games, photographing my dinner, linking FB to Twitter or MN, telling everyone exactly where I am in the world at any given moment and with who, and all that malarky.
I have a few MNers on my FB but not too many. I can tell by looking at the friends' lists of the MNers I have on mine that they all have other MN/FB friends in common and it bugs me that I can't work out who they are on here.