It gets used online for things, and there are whole communities devoted to shitposting, but I can’t find a clear set of rules for something to count as a shitpost. I remember querying whether a post on a shitposting community was witty enough to be a shitpost rather than just a shitty post, but of course not all the responses to that were terribly helpful!
A shitpost is something low effort, that simply takes up space in the content feed without being engaging, entertaining, or interesting.
One of the primary forms of shitpost is a meme or joke that isn’t very funny, or has been reposted to death. So “shitposting” can refer to the act of spamming shitty meme posts.