I don't have a good answer to that question, but I can say that it hasn't improved mine.
The same type of f***-ups I make in person, I've also made on Facebook. Maybe worse even. Because without real-time feedback of the appropriateness or inappropriateness of my diatribes, I go off in directions that I would otherwise reel back from in person.
Plus, when I'm overwhelmed by fatigue, I tend to write letters of a very personal nature, which can make people feel uncomfortable. And while sending a long Facebook message isn't any different functionally from writing a long e-mail, people can unfriend you on Facebook. And that hurts.
I think that Moog is correct in saying that trying it just as an experiment might not be the best idea. Someone suggested that exact thing to me a month ago, and now I feel worse than I would have if I hadn't hopped on. If you're going to try it, do it because you have some friends online that you want to talk to or keep in touch with, not just to arbitrarily test social skills.
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"It is a dreadful feeling to come back... and find not a kindred soul, no one who cares whether one is dead or alive." -- Emma Goldman