Anonymity on Facebook - how do they suggest people I should know?

So, yes, they appear to have a deal with the Telecommunication Providers in different Countries.

Well that's ONE explanation.

Another one that I like better is simply that they have all their users' contact lists, thanks to their mobile application which no doubt reads everything and sends it back to their headquarters.

All they have to do after you register with your real phone number is look through all those contact lists, and find the people who possess your number.

This idea that they may have arrangements with telecom providers seems a little far-fetched to me, in great part because it is simply illegal in many countries to disclose phone records to anyone without a court order.

Repeat the same process, but use a new prepaid phone number. If they can still guess who you are then it is freaky. If not, then it is probably your friends' contact lists which have been sucked up into Facebook (not so freaky, just regular FB creepy).

It would be an interesting exercise to try the same, but with your work number and see what kind of connections Facebook infers from that.