Mike Monteiro has some good, simple advice for people who use Twitter:
Never, ever, talk about clients on Twitter. Ever.
I pretty much take this one step further — never talk about other humans with whom you have a personal relationship on Twitter. I talk to other people I know on Twitter, but not about them. I try hard not to even make oblique inside jokes about other people or the code they write. It’s unprofessional and rude.
If there’s one advantage to having been online essentially forever is that nobody had to teach me this stuff after I was a grownup. I came up through the world of dialup BBSes, so I had already confronted the implications of social media long before sites like Facebook and Twitter even existed. I was there when people started getting fired for things they posted on their blogs. In some ways, I feel bad for people who have never been online and jump right into it with Facebook or Twitter. The great danger of social media is that it creates the feeling of intimacy when you are, in fact, on a potentially global stage.
My basic philosophy on social media can be deduced from my old post on levels of candor.