I use a pen name sometimes, mostly because of divorce. I started using "katywhowaited" here when I first started posting some old love letters from Vietnam written by my former husband to be protective of the feelings of adult children of his new family and mine who might happen upon them. Now that I'm close to 70, and they all are at the same age I was when my husband left, I don't think I have to do that any longer. (By the way, using alternate names here is NOT a good idea even though it's allowed because it divides your statistics in half!) Sometimes I use my old name before the divorce because I was married for almost 20 years, and that name is on a lot of my writing.
For the children's book I recently published, I used this name (my maiden name) because I had some kind of nostalgic feeling of wanting to go back to my roots. (It's also "prettier" than if I use my new husband's name).
I use another COMPLETELY different name when I quote myself (tee hee) on facebook - especially if it's political or if I want it to sound like a "real quote" from someone important. I use the initials jd with a last name for that because a man still seems to have more credibility than a woman I think. I especially write only my truest politically left ideas with that moniker because I have dear, dear friends who are very conservative leaning in politics and religion who would be offended by my viewpoints. I love them dearly and understand their viewpoints, but would offend them with mine.
My favorite story about my multiple identities is the time when I was writing a comment on a comment at Huffington Post. I really was totally in sync with everything this person was saying. Just as I was reveling in the fact that 'FINALLY, I found a soulmate in my thinking,' I looked up and figured out that "Katy" who had posted the comment I was commenting on was ME. (It's nice to know you have a fan club, even if it's only you.)