No one remembers Dopplr? It was a pretty popular social travel planning site before Nokia bought them and froze it to death. It was like, one of the original web 2.0 sites alongside Flickr.
This was in Germany and a few years ago, though. With only tech people. Not a bad demographic, but there's probably a reason it got canned.
But it cracked OP's issue of needing daily use so you remember the service. I would check Dopplr to see which of my friends were coming to London -- that's much more frequent than me travelling, to the tune of travel_frequency * friends / cities. It was useful, and I still miss Dopplr today.