I do this as well. My Facebook community takes up a lot of various small circles that I interact with, including family, a long-term e-mail list serve membership, high school friends, and a former circle from a Usenet newsgroup (rec.equestrian) that was controversial in its own right, back in the day. Throw in a dash of old political connections, tiptoeing into teacher connections, and I definitely don't want to have the crazy intervene on Facebook.

Gotta say, it can be somewhat entertaining when the different groups start talking at each other. Like, say, the rec.eq crowd (rec.equestrian was not once known as "wreck.eq" for nothing, and my rec.eq connections were gleefully once part of that batch that made the wreck what it is!) talking to the listserve crowd, or the sf crowd. Can be entertaining.