Facebook, I can see at least, they trick you into spending a lot of time on their site with flashy useless games, questionaires, they have reasonable features to share pictures with comments, and they silently hold all your data hostage, which no one really seems to realize or care about (they are happy to import your data, but try getting it out, like even getting an rss feed of your friend's updates. Not gonna happen, they want to force you to use their site for that).
Oh, and of course facebook also has the oh so mostly boring twitter like status messages about whether someone I know had one or two eggs for breakfast, or what they think of the day's traffic, or rain, something I really care about :)
(yes, some people do a better job of posting more interesting things overall, but unfortunately they seem to be the exception more than the rule)

But back to twitter: I've been reading jbq's twitter via RSS out of curiosity since twitter seems to suck so much that they make old tweets unavailable after a certain number of days (not unlike facebook which also silently drops updates from friends if you didn't come to read them often enough), and the only think I can see that's remotely useful about twitter is people saying
#footopic: what I think about it

so that you can quickly search #footopic amongst all people who tweet.
But twitter being used as a poor (and crappy) man's IRC by saying
@tweetname: my reply to your tweet

which is then for everyone to see and mixed up with maybe somewhat worthwhile tweet updates in the middle. WTF? Seriously, WTF???
And more generally, if there is some kind of barrier to entry on posts from people, you get better content overall. In other words, if you can tweet whatever goes through your mind with no effort at any time, the overall output is mostly random useless and/or incoherent thoughts from random people. It makes the random blog look good in comparison :)

Oh, and back to facebook for a minute, let's just say for a second that I wanted to put links to my blog posts on facebook for people who think that facebook is the new internet and don't seem to know what an RSS reader is for, 'Blog RSS Feed Reader' tells me "Allowing Blog RSS Feed Reader access will let it pull your profile information, photos, your friends' info, and other content that it requires to work."
Err, what??? That's the worst part about facebook, they share all your info and your friends info, even when it's clearly absolutely not needed like an rss reader that is bringing my blog into facebook. And of course, the worst part is that any of the people who are 'friends' with me on facebook have probably shared whatever little information I gave facebook without my consent.
That's just a crock of shit. I'm happy I didn't give any relevant information to facebook and lied about my birthday so that it can't be used for ID theft later.

If you didn't know, search the net for how facebook does their best to keep your data hostage, and to force everyone to spend as much time as possible on facebook as opposed to reading outside text that had the gall of not having been published on facebook.