Pardon the expression, but people are complete suckers for sort of thing.

Given two similar services, one with a limit on registration and one without, you can bet that the one with the limit will garner some sort of popularity just by virtue of being "exclusive."

This doesn't mean it's a bad marketing tactic. On the contrary, it's obviously excellent for exposure... but if it ever becomes a popular service, saying "I was there first" is a huge deal for people. For reference: Lines outside movie theaters days in advance for the Star Wars movies, video game midnight release parties, and 8 hour lines outside Apple stores for a phone.