this may just be a personal opinion but for myself the main drive to register to a website would be a giveaway.
that could be the best way to drive traffic towards your site. unless you have a huge budget i don't feel spending money on say google ads will actually help you.
even giving away $25 would drive traffic towards your site. lets say only 5-10% of these people stick around i still feel this is much better than the results you would see on google.
I often hold contests myself, I haven't had any since I'm working on a revamp. Though at one time I did give away a Wii U console.
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I offered a £25 prize in a referral contest throughout March/April, but appeared to have no interest so opted to withdraw it..
Maybe it was too soon as I only have a handful of members, or maybe the prize wasn't enough. Either way I thought it was a good idea to try and get new signups and it didn't work.
I might try again in the future, time will tell.
Nerdface
I haven't failed. I've just found 10,000 ways that won't work.