Mostly, that's because it sells ads.
In 2012, those ads generated almost all of Facebook's $5 billion revenues.
Why do these ads work when they work?
There are two main reasons.
One is that they are well-targeted. Facebook ad buyers can tell Facebook who they want to see their ads.
The second reason Facebook ads work, when they work, is that the ads are well-made.
What does "well-made" mean?
We asked Facebook's research team, and they provided us with some answers.