Everybody who creates solutions where interaction with other users is a part of the service should read about ‘The Engagement Horizon’ in this blog post.
The number of services I have tired but never used again is significant. The reason has often been the fact I was a lonely user without somebody to interact or engage with. Trying a service in the category of ‘See what your friends….’ or ‘Collaborate with your colleagues’ and then just watch an empty screen/map/list is not a way of making users come back.
I expect many services have gone to the dead pool for this simple reasons, they had users, but it was lonely users not clusters of connected users.