You stay in their infrastructure. The goal of Facebook is that you do not leave it. Some people think the Internet is Facebook.
But they also know that you want to keep talking one to one to your friends in private. Else you use Snapchat or worse. Your mother, colleges, and boss are on Facebook too, right? But they need your info for their timeline. This is what they sell. As long as you do something with your friends or alone and post about it later, they don’t loose much.
Loosing you to another service is worse (from their perspective).
If you want to learn more, read about Facebook’s upcoming F8 conference or check what’s going on AI-wise.