Generally, well-known (internet-wise, that is) acronyms are okay, but in limited usage. Overuse of them in several short posts borders on chat-speak.
There's no exact number of how many is permitted or not, but from my experience, it seems to be that if you are already spelling out words, punctuating, and spelling as best as possible, then it is okay, OK?

From the likes of your posts,
nomis3613, I don't see a problem with that.
You also have to expect that not everyone will know what "FWIW" means in a thread conversation; thus, you have to be understanding in return. (Not that interrupting the thread to save 30-seconds of Google-time is any more acceptable a gesture.) If the thread is long enough, it is sort of expected, rather than typing out the same couple of words all the time.
There's also so many that are used so often within all of our specialist discussions of gaming, automobiles, local terminology, and technology that even we as moderators forget that they are almost commonplace, which can make even the most manicured and finely-written posts illegible to someone not in the know.