This is an ongoing thing that's been happening for years. I don't think anyone knows who started it but it's common knowledge that December is the month to make a Pokemon your profile picture. I've been doing it every year for the past 3-4 years.
Yeah I do it. At midnight on the 1st I switched over to a Volcarona cause that's my favorite. I think in a few days I'll find another picture of it and switch to that. I believe I switched to like 5 different Pokemon over the course of the month, judging by my profile pictures album, haha. But now I'm firmly obsessed with Volcarona so I'll just use a bunch of badass pictures of him.
I rarely ever go on Facebook, so no. Even if I used Facebook regularly, I don't think I'd do it due to having only one or two people on my Friends List knowing what Pokémon is, and recognising that it's not just for "kids".
I haven't... but I might just do so now. Hold on a second... yeah, it's up. Tornadus, actually. Did it last year with Hydregion, and the only reason I didn't do it immediately this year was because I hadn't had my profile picture as myself in a while and wanted to keep it up longer. Still, I think it's fun, and I don't mind everyone seeing - all my friends already know I'm a Pokemon nut.