No, I didn't buy them. I got some Facebook credits for free. (It may or may not have involved a zany scheme.)
So what exactly do I do with them? I'm stumped. Facebook credits appear to be completely worthless; there's nothing you can buy with them (except "virtual goods" which are inherently worthless in that they don't exist and you can't own them).
I've bugged a phone number for Facebook out of a harried Zynga support agent (who considered my email to be proof positive that I could be persuaded to actually play Farmville if only s/he threw enough powerups at me) so I can call some Facebook support agent and try to cash out my Facebook credits for hard currency. (If I let Facebook keep the 30% cut they charge developers and only ask for the 70% they'd pay out anyway, they might agree to it!)
If not, maybe I can register as a developer and create a Facebook app that lets you look at pictures of cake if you pay me money, then use the app to cash out my credits. And, with luck, the credits of millions of Facebook thickies who think my app is the COOLEST THING EVAR!!11!eleventyone!