So Twitter is too much and LinkedIn seems increasingly impersonal.  But Dopplr -- just right, at least for now.  I like the little thumbnails of people, I like that I can see who's in the town I'm in, and I like the simplicity.  Not a lot of bells, whistles, and meta-bettah-information - at least for now.  It's like...a calling card system for people who move around a lot. 

Perhaps you are unfamiliar with the practice of calling and leaving cards.  I know about it only from my (now late) grandmother and from a 1920s etiquette book that I bought at a garage sale once.  Basically, the idea was that people established At Home afternoons when they were available for calls.  But you would only call on people you already knew.  And the first time around, you would do no more than leave your card - then the recipient would decide whether to return or acknowledge your call.  Complicated - sometimes the cards would have corners turned up to signal particular messages.

So you start in Dopplr by being invited (just ask, lots of invitations floating around).  Then you ask other people if they want to be fellow travellers of yours.  Then they can see your trips, but they can decide in turn if they want to allow you to see theirs.  Just like the card system -- polite, signals sent and received, levels of acquaintance decided on -- but faster and spanning the globe. 

The visit that follows the signal - that's the interesting part across which a discreet veil will be drawn.