The entire Internet can be a huge productivity sink as well, so I don't really consider that a proper negative. I do agree that Facebook isn't perfect of course. There are some weird issues that arise out of Facebook where people feel offended over petty issues like "oh, you didn't write on my wall on my birthday! how dare you!" But again, I think that's just exposing said person's innate pettiness.
Or rather, it's a powerful tool that let's people accomplish Good things more efficiently, but Bad things as well. But so can lots of things. And it all comes down to intended purpose. Right now it's being abused because its new and people don't know any better, but that happens with everything. The Internet started off as being ridiculous with porn, and now porn is a minority on the net. Chat rooms used to be soooo evil, but now people know how to use them properly, and they can be quite useful. They have evolved into the IM apps we're seeing today, and while those can be counterproductive, they're also extremely useful as well.
So just because it can be used for evil doesn't necessarily mean it IS evil.
It's just a tool.
And I love it!!!