I have Trello account and tried to use it when it launched but eventually I reverted back to my good old pad and pen once again.
Trello has been a big hit with my wife who is a writer. She uses it to organize scenes, chapters and various snippets before integrating them into her draft. I've tried introducing various storyboarding applications to her with very high resistance -- Trello was the only thing that really "took" with her.
If you just want to jot down notes and don't need to organize them, Trello's probably just a bit too rich an interface. Once you've got more balls in the air and jugglers in the mix, Trello becomes a lot more interesting.
Signups to free services matter less, relatively as well.
What means something though, in nearly everyone's mind, is a large number. 500k free signups to a service (regardless of whether they login more than once (which is what I did btw) is more newsworthy and attention getting then a small amount of paid users paying bupkas to the traditional world. (I'm not talking about someone posting on HN about something that they put together over the weekend which earns them $1000 per month which HN finds interesting).
We track a bunch of other interesting numbers as well, such as number of active users in the last month, number of users with four actions (they actually clicked something) in the last month, number of new boards created in the last day/week, etc.