How to integrate FB javascript API with my backend?

I am using Facebooks javascript SDK, and I am able to login and do everything with it.

For backend I just have a simple rest api with database behind.

This is the call that gives me the status at the moment: http://ift.tt/1m1d46x.

Question is, when login and status checking are done on my front end side, how does my backend know which user is logged in?

Yes I can just send the username to my REST endpoint, but since users can manipulate the ajax calls, this is quite unsafe. How should I be doing this in real world?