I had the feeling no one used Dopplr for vacations.
Me and quite a few people I know input their conference and business trips, so with only a bit more traction it could have served "oh, you're also in $CITY, let's meet" quite well.
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.
Dopplr went after the conference/business market, and at least in my crowd it had critical mass. That might be because I know the founders...
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.
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.