Activity mining refers to people making one-line vaguely-on-topic (just enough so that they don't get deleted) comments on the forum so their activity and rank increases and they can sell their account for a higher price. The people/bots doing this aren't really paying attention, so they usually don't participate in the conversation or answer questions. It is usually best to just ignore them although reporting to the forum mods is an option (they are overworked and have to prioritize more serious abuse so this often doesn't accomplish anything). The practice is rampant and has been going on for years.

(previous comment above this one also)