That's why (if addicted to SM) you only need sign up for free (hardly a task) - and then just share with friends that you have. Check back in a few weeks and see if others have signed up. When enough have, then start using it, until it is good enough to dump the other.
I don't use SM apart from this and a few dedicated forums, but I don't get why anyone would be on any of them to begin with, regardless if my friends are on it or not.