Question

Inspecting what facebook is doing in my navigator, I see this code:

for (;;);{"t":"refresh"}

If you try to evaluate it, you can figure what happens (infinite loop).

Do you Know what it is?

Answer 1

Am I correct if this was inside the response from an ajax call?

This is a one of the strategies employed to avoid XSS when dynamically adding scripts that contain user-specific content.
If it had not been here, an [evil] page could have requested this script inside a regular script tag, and have access to the methods and objects defined by it.

The code loading this script from Facebook using xhr will remove the first section before evaluating it to get its content. It this case the result would be {"t":"refresh"}.

A script from a different domain is not able to do this.