If you are finding it hard getting sign ups for your sites then you may consider buying referrals. There are quite a few sites where you can do this for different prices. It may not be as good as signing up a friend that you know will definitely be active but at least you know that you will not have to do very much work to get them.
If you want bargains then you can buy exposures for your site or referrals from Buy Refs. They charge between $0.24 and $2.49 per referral depending on how many you buy and whether you want USA or International sign ups.
Get Ref is a website where you can buy site exposures and referrals too but you can also earn credits to spend on the site by referring members and clicking ads.