The two platforms are completely different. The users of FB (and LinkedIn) put their real information on the website, allowing advertisers to target effectively which raises the average ad value on the website. Intuitively, revenue = ad value * # impressions.

Twitter is in between anonymous forums (unfortunately they all died) and FB/LinkedIn in terms of ability of advertisers to target users. They're living off of high impressions, and they've basically already saturated the market. Now they need to somehow raise ad value, which would mean integrating hundreds of free, quality services to gather data like Google or tying Twitter accounts to real identities (like the Google+ attempt)