Just suddenly powering off on you (not rebooting, not displaying a system halt/blue screen error) points to a thermal issue. Not surprising that it would happen on Facebook given the amount of resources it consumes, but it would probably be expected to happen on other scenarios too while your system is under heavy or prolonged load.

Is this a Windows machine? If so install SpeedFan and check what your temps are.

If your computer is momentarily flashing a blue screen error before it dies on you, then there’s a higher potential for it to be a memory-related issue. But for a sudden power off without a stop error, thermal is the leading candidate.