I guess you are wondering because of those commercials they run once in awhile showing people meeting that way.
Actually, it's not common because one would have to leave his or her BT open to receiving communications. Not a good idea.
Here in San Francisco some bars and cafes were having PDA/Cell phone blue tooth events. People would send text messages to those whom they were interested in. This was a Y2K version of those bars that had telephones at each table. Singles would call the number posted at a table to express interest.
Far as I know the whole thing fizzled. I think because it was too difficult to get the individual devices to work. The news showed some guy running table to table trying to set up the devices to communicate.
Actually, judging by who was in the news clip, I think only geeks showed an interest in the concept. Assuming the typical geek profile, you can figure out the real reason the idea failed.