Sharing does increase the chance of new members joining, since you're sharing it with your friends (average: 200+ people) however a lot of people don't want people to know they're playing RS, or atleast they don't want all their friends to know. I'm OK with saying I used to, or still play RS sometimes if it comes up in a conversation with someone, however I'm not ready to share I'm on OSB and for them to be able to check me on here to my ~550 friends I have on facebook.
Hope you can figure something out