For example, if you make a habit of being rude and obnoxious on sites like Facebook and Twitter it could effect your personal life, or even your career. Everything you post on such sites is findable now.
But being well mannered is also good for business. You just get more followers, fans, likes and retweets, etc if you are polite and helpful. And if you're widely perceived as being approachable as well as knowledgeable, there's a much greater likelihood that you'll receive interview requests and guest posting offers.
Here's just one little example of how being polite has helped me: Recently I posted a question on Linkedin Answers. I got several responses. You can rate the answers as "good" and "best", which I did. So I assume those people were notified.
But there were others I hadn't acknowledged. I hadn't used this facility before and thought there might be some sort of automatic thank button, like on Quora. But there wasn't. So I sent e-mails to all those who'd answered my question, thanking them for their valuable input.
Pretty soon after that one of these people, who's very well established and has a big network, sent me a connection request, which I immedaitely accepted. One of the other users who answered -- again with a big network -- e-mailed me to say he was happy to offer his advice, and not to hesitate to contact him for any other questions. So, a conversation has started there.
Such developments are hardly remarkable, of course. But I wouldn't have made those connections had I not made a point of saying thanks. Clearly, being polite on social media can result in immediate benefits.