[NightWish]

I think the meaning of "meta-posting" is generally best understood by comparison with a more commonly used term "meta-data", which is commonly defined as "data about data". In other words, "meta-posting" is "posting about posting". It is one of many almost-neologisms that get created by using the "meta-" prefix.

Originally Posted by Joojoobees

[P]osting about whether people should be posting

Meaning, yes, your definition is correct in as much as it is indeed an example of when someone might be meta-posting.

Originally Posted by Solace

Meta posting is when people try to act like a moderator

Indeed, that behaviour is another very a common (most noticeable?) example of meta-posting. By definition it is a moderator's job to moderate a discussion, which often requires them post about the discussion (other posts) taking place, in the middle of the discussion. Hence posting about posting. Other user's doing this has all the implication outlined in Solace's post.

It does go wider than both of those though; it doesn't have to be instructive (as might be implied by the expression "should be" or "act[ing] like a moderator"). Simply posting to say you liked someone else's post is meta-posting. Saying "wow, we're up to page 100" is also meta-posting. Any time you post about a post, or the thread in general, you move the focus onto the posts themselves rather than the thread's topic.