I consider people spontaneously tweeting or blogging about it to be huge. I know it may seem trite, and as everyone loves to point out, it's not revenue, BUT i

I consider people spontaneously tweeting or blogging about it to be huge. I know it may seem trite, and as everyone loves to point out, it's not revenue, BUT it does mean they are putting their own reputation on the line that their friends will like it. I also have metrics within my app itself. Like out of the people who want to signed up, how many completed useful chunk in the application. Out of those, how many took the next step. So far I haven't spent too much time looking at bounce rate and repeat visitor rate, but those are valuable metrics too. I also use UserVoice and it's been really great. A lot of people have taken the time to post something (and that in itself tells me a lot about how people feel about it) and their comments feed me energy to keep going and making improvements. I also put my email address right on the account page (at least during the beta period) and invite people to email me feedback. A lot of them do that too and I always respond and get some sort of useful feedback (beyond just a bug report).