Stack Overflow benefits from network effect, and in the "sharing economy" of people contributing their knowledge in exchange for "internet points". Thus, you do generate value (not necessarily monetarily) off of even free users, if they contribute to discussions.

(I can't speak to Trello as I haven't needed/used it)

I'd argue that the success of the IAP model used in the app stores is much more of a response to the lack of free trial versions, which is kind of like the drug pusher "the first hit is free" sales method...