Hint: if you are starting a social network, let people sign up as soon as you go public. The number of users you have is what will make or break you. Denying immediate entry is turning away your customers, most likely for good.
I don't see "lack of ads" as particularly craved by users. My wife actively wants to see ads (to the suffering of my credit card). Unless they find something else to latch on to, this will end up being yet another also-ran social.