Facebook's product is you. Your data, habits, communications. Twitter is a simpler model. It costs $360k/yr[1] to get access to only 50% of all tweets. Combined with advertising they finally turned their first profit in Q4 2013[2].

Imagine the value of the more intimate data facebook has about you.

[1]http://readwrite.com/2010/11/17/twitter_to_sell_50_of_all_tw...

[2]http://www.ibtimes.com/twitter-nyse-twtr-earnings-q4-2013-tw...