a law would mean it has to hold true everytime, for everyone. I certainly laughed when I saw this, as did eldoraro and nfs4
No, a law is an observation of phenomena. So Newton's laws of motion are still laws even though they breakdown at certain extremes.
A theory attempts to explain phenomena. That's why the theory of evolution is still a theory, even though we are pretty damn sure it's correct. It is an explanation of the diversity of life.
Oldsmoboat's law is a law because he observed that when people promised lulz in their thread title, as a rule, the OP didn't deliver.
FWIW, I've seen this image before (including the Facebook caption) and it does make me laugh every time.