Writing posts is a form of socialising with people so you're going to get at least some benefit. But the match-up to real life is hardly exact, so that benefit will be limited. Hard to say how much though.
I think the thing you can work on the most is polishing the things that you say. You can practice your humor, telling a good story, and things like that. Does that make sense?
You can pick up some bad habits too though. Many forums can be subtley or overtly negative. This can start to get to you after a while and affect your thinking. They also emphasize debating and arguing your point. Being too much of a 'debator' in the real world can get tiresome with many people.