Hi David -- There are a few sites like that. You could try travbuddy.com if you are interested in meeting travelers by location or showing them around your hometown, although it has less of a tech slant. couchsurfing.org has more of a backpacking focus, but they also have regular meet-ups around the world.
I travel occasionally, and in reality, it's not my circle of friends that needs to be informed, it's the people that I could potentially meet somewhere. For instance, I'm part of Debian (although I really ought to officially retire) and the Apache Software Foundation. There are lists where I can mention that I'm going to be somewhere, and more often than not, it turns out there's someone from there who is happy to meet up. Or the other way around: when people visit Padova (or wherever I happen to be living), I'd be happy to go out and chat about whatever topic we have in common. I want something that lets me get in touch, in a casual and no obligations way, with people who I have something in common with. I suppose that it's never going to be perfect because you can't force all the people from various lists to sign up for some service, but I think it could be improved.