My Tweetfave service has been running for just over 4 years now and just passed the 500K tweets mark! The usage has been pretty steady considering I haven’t done much to promote it. Over 270 users have tried the service, with about 100 still active. Luckily the service hasn’t required much maintenance over that time, just an occasional update to deal with webservice API changes or to fix minor bugs.
Here’s the growth chart covering the last 4 years:
If you use Twitter and use the “favorites” feature, give Tweetfave a try. The service will automatically email you (every couple of hours) all the tweets you liked.