That some way to state the fact. I'm sure OP understands but, if I may;
Is it only posts that counts towards your being active and counts in the activity meter?
I thought sending pm did matter too and counts towards have been active.
You ask in a different context. The activity op is asking about is the "activity count" reflected on the profile page which affects rankings. The activity you mean means nothing if you expect to rank up, even you are considered active when you log in for the first time without doing anything.