One guess is a thermal shutdown. Is it a desktop or laptop? Are the ventilation ducts unblocked? If they are, or if the CPU fan is broken, then running an app that pegs the CPU could cause a thermal shutdown when other apps don't. Sometimes, ads served on a particular website are prone to pegging the CPU (FT did that for me for months, but now seems to be behaving). Check your CPU usage in Task Manager when you go to Facebook, before your computer shuts down, and see if it's high.