There are a lot of "social" travel sites emerging. Dopplr is one popular one but my favorite, for one simple reason, is TripIt (http://www.tripit.com). That one reason: they've solved the data input problem. Most travel sites require that you enter full details of your trip (flights, hotels, reservation numbers, etc.). TripIt however has you send your confirmation email to firstname.lastname@example.org and it automatically parses the email to figure out the relevant travel items in the email.
The social features on all of these sites are still evolving and TripIt's not the best of the breed but if only because it solves the data entry problem, TripIt is my first choice. Dopplr seems to have a lead, at least among my travel network, but I'm trying to move them all over to TripIt. At the very least, given the lack of data entry, it's reasonable to use it as your second travel site. There's minimal incremental effort and some added value.