Facebook, with few exceptions, allows you to pick your team. If you really want to work on something, you can. As for your last question, I joined a year ago, and from my experience, the meritocracy is extremely alive and well. You can go far, and get there very quickly, if the fit is right, and you are kicking ass.
Reading between the lines of your posts, I think you might have a pretty inaccurate view of facebook, as it currently is. Facebook has fewer engineers than you think, and it's -really- easy to have massive impact on features, or even the org as a whole, from day 1. Most of the teams responsible for major features/products are quite small. Some are frighteningly small. In many places, there are huge, ambitious, crazy ideas currently on the shelf waiting for 2-3 new people to join the team to take on.
I like your work definitely. Guess a lot of engineers would; I'm curious to know how much significant work you'll be doing if you apply now (jump through the hoops), get in and want to work on HipHop (as recent graduate). What are your options within Facebook; how far can you grow?