I think “Last Seen” is a more useful data point than “Last Post”, at least as far as the profile card pop-up is concerned. I propose to swap out “Last Post” for “Last Seen” in the profile card.
Take the user @Cafeine for instance (sorry about the unmerited mention mate )
If you find an old discussion with a post of his that you might want to address with a reply or private message, you might think twice when you see his profile:
By the looks of it, he hasn’t been around for a while, so your chances of a response from him seem slim. However, if you look at his profile:
“Seen 3 days ago”. So he actually checks in every once in a while. Might not be such a futile attempt to get in touch with him after all.