Some, like these, are not specific to FlyerTalk and are used more or less universally across all matter of Internet communications media (chatrooms, IMing, other bulletin boards, etc.). I'd suspect that the good folks who have contributed to the FT Glossary were focusing more on the acronyms unique to FT and may have assumed folks were already familiar with typical Internet acronyms (e.g. BRB, AFAIK, TTFN, ROFL, FYI, etc.).

There are numerous sources online for such acronyms--here's one I just found on Google.

That said, it might not be a bad idea for the FT Glossary to put in some of the ones most commonly used here.