My pain in the arse ex found out about me through facebook too the other week. I never knew people read those things, but apparently they do. I didn't change it until I was ready to be out, I used it as a way to come out without having to talk to people I rarely see 1 on 1.
Changing your facebook is akin to a public outing - but I don't have to tell you that now!
Your parents are going to figure something is up eventually. They usually know you better than you think. More often than not, they are just concerned for your mental wellbeing.
If they ask, I'd just explain it as you have on here. You're finding yourself, and trying on a few different names and ways to relate to yourself. It isn't unusual for young people to do this, we all have to find our identity - trans and non-trans alike. Identity crisis is a great short way to explain things without getting into the finer details.
If you don't want to hint at what the deal is, you could always say it's a nickname you like, and you prefer to be a guy on the internet because it's "safer". Parents are afraid of internet preditors and identity theft (just as the TV tells them to be) so if they're not so internet savvy, they'll dig it. I don't reccomend outright lying though if you intend to come out down the track.
Good luck, I hope it all works out for you, and your family handles it with patience and understanding.
