Planning to turn this into a top ten list, but here is a start.  The order may take some work too.

1. Your name becomes used as verb or noun.  Biggest success here in recent memory is of course “google”, but it was nice to see a “Hail to The Twitterer” headline in the New York Times this weekend (print edition, online the article has a different title).
   
   

2. Your friends’ grandparents are using the service.
   
   

3. Your friends’ parents are using the service.
   
   

4. Hackers and spammers are targeting your service.  Here too Twitter has made some recent “progress” as uncovered by the folks at Kaspersky.
   
   

5. You overhear random strangers talking about your service in the elevator, on the subway, in a bar *not* located in Silicon Valley or Alley.
   
   

6. You have a meaningful number of active users in Kentucky (for services targeting US market).
   
   

7. People recommend your own service to you.