social networking database design

[Usman Sookia]

Hi,
I am designing a soial networking website. Is there anyone who has some experience with database design in this context that can provide me some advice about how an optimal design should be?
I have searched on the internet and the only advice I obtained is to have one table of accounts(id, other_fields) and a table for relations(id_account1, id_account2).
Bur normally, there are some typical rules on social websites (that are also requirements of my project) like Person X can inmail Person Y for free if X is friend of a friend of Y (N2 - relation) or for Z euros if they share an N3-relation and not at all if above N3.
I'm afraid that with the above design the system should take too much time to do such verifications before for example displaying a message to the user that he cannot inmail or even verifying if he can inmail.
My business rules are limited to N3 relationships (X is contact of contact of contact Y). The rest are treated as > N3.

Thanks for your help
Usman