$1 Billion in profit => $1/(user year) in profit for 2011, correct?

That's pretty darn good money, but not a huge amount more room for user-growth. They'll grow by having new businesses/making current ones more profitable. New business would have to be monetizable (eventually, at least), current businesses would either have to convert better or become less costly to run.

Facebook is full of some smart cookies - it'll be interesting to see how they accomplish those paths.