I am not able to search for any question or even a keyword @ facebook.stackoverflow.com
For anything I search for, its giving : Your search returned no matches.
Is site under maintenance?
Apologies, this was an oversight in some search code. As stated in comments above facebook.so is a subsite (it doesn't have it's own database)...nor does it have its own search index. It works off the parent index via a filter at index time, but wasn't correctly pointing to that index.
This has been corrected and a build just pushed out.