Unless OKCupid is hiring the government or people with guns to threaten other websites out of existence, there won't be a drift towards a monopoly.

A monopoly isn't created by one company getting the overwhelming majority of customers. A monopoly is only created when competitors cannot enter the market. It's a subtle distinction but it's very important, because what's implied is that the company with the monopoly can jack up their prices and abuse customers. They can't do this without feeding a garden of small competitors that can and will outgrow them (see Myspace, America Online, etc), unless those competitors are disallowed from ever existing.

You can keep downvoting this, but it's a very important concept in economics and it will still be true.