Did you seriously just say that "paid advertising rarely works?" Really? You believe that the entire 50 $billion Internet advertising industry is just wasting its money?
I could have been more specific, but paying for advertising on OKCupid for this startup in particular is just not going to work. It's not cost effective and very few people will be clicking through on those ads.
I guess the better way to put it is that if your startup has to resort to paid advertising as the main user acquisition funnel, you probably already lost unless you have ridiculous conversion and LTV on each user.
I think that "paid advertising rarely works" is actually true and yes, the whole (Internet or not) advertising industry is really wasting tons of money - and more importantly, fuel and labor.
The thing is, marketing is mostly zero-sum game; people are stuck in a negative feedback loop of throwing more and more money to one-up their competitors at attracting customers from the same limited pool. It's an unlimited resource sink, but it's a) what you have to do lest you get outcompeted by others, and b) profitable for those who facilitate advertising.
Weixiyen is also right that this "growth hack" is effective, but remember that "growth hacking" is mostly a fancy term for spamming / abusing trust. Every new way to trick people into buying something will work for a while, until everyone else starts doing the same and the ROI drops to zero.
Once upon a time, a pop-up banner was the clever growth hacking trick. As HN's idlewords tells the story,
"One of the first banner ads had a click-through rate of 78%. That's mind-boggling. Do you know what two words were on that banner ad?
"Shop Naked!" CLICK!
But banner ads turned out to be like poison ivy. People clicked them once, and learned never to touch them again. The industry collapsed in a flurry of pop-overs, pop-unders, and shame."