I think the retention mechanism is built in. If you want to play, you can do so without registering. However, if you want to get messages from other people, o

I think the retention mechanism is built in. If you want to play, you can do so without registering. However, if you want to get messages from other people, or to follow up with people later, you'll likely end up registering, and then I'm sure random messages will end up bringing you back.