One of the things that frightens me with social networking and Facebook in particular is the fact that it wants you to connect it with so many other sites. Given the recent hacking binge by LulzSec proving how inefficient a lot of Internet security is, the idea of linking everything to one site, one site that houses personal information seems like a really bad idea.
Besides that (slightly) irrational fear I also don't like the way it makes people think they're activists. "Put a green tint on your profile picture to show that you support Iranian freedom" or "Put up your favorite cartoon character to help end child abuse" stuff really grates me. You're not doing anything to really help a cause by changing your profile picture or liking something.
Twitter is perhaps the worst offender for this idea of "slacktivism" (see what I did there). Here are just a few articles about the media dubbed "Twitter Revolution" in Iran back in 2009 that showcases what I'm talking about.
This article talks about people just changing their location to "Tehran" in order to act like they were taking part in the revolution
This one talks about Westerners tweets and facebook updates causing trouble for the people actually living there, as well as Westerners retweeting propaganda posted by the Iranian government.
I'm not saying that social networking activism can't be a good thing, on the far side of the spectrum Facebook was used to help group together volunteer cleanup crews after the Stanley Cup riot in Vancouver this year. Not only that, but a lot of people helped the police find rioters by posting photos they took, tagging people they knew, and taking screen shots of incriminating facebook status updates.
It definitely works on a local or state/provincial level, or maybe even a national level, but as far as international, it just lets people feel like they're taking part in something when in actuality they're not, and I think that instills people with a false sense of achievement.