in this sentence, keep means= make sure posted= updated, aware people say "posted" because when you want to make others aware of certain information you "post" the information on the bulletin board or wall

"Keep me informed." They don't need to tell you directly, they can send a message as if they post you the news.

Keep me posted = Let me know any news on the matter

Thanks you all for your answers!

"keep me updated." :)