Just a thought. The value of all our social networks are the members. I have set up my site so that members are hidden unless you log in, that's a great start, stops lurkers rifling through members, at least they have to show themselves before they can do it. So now two thoughts...
1. Can you hide members behind a paywall, like I say if anything on a social network is worth paying for, it's to see and 'friend' members
2. Could you limit the number of searches a signed in member could do, so only 3 searches per day.
I knoe point 1 may be a bit counter intuitive, why join a network if you can't see the members, but my thinking is you join to be part of the network subject - so in my case because we are a business network, you will get access to lots of local businesses, but equally if it was a horse loving site, you would join because you love horses not because you see a particular person.
Any thoughts, it's an unusually warm day over here in Chorley, England so I may have sun stroke ;-).
While I like the idea of forcing people to sign up in order to see current users and search for new ones, I feel that these search limits will be heavily detract from your network. Once people are using your site most are going to want to be able to do anything the site is capable of.
IMO this would become annoying and cumbersome very quickly and I might not use the site if your put down such harsh restrictions right away.