Services like OKCupid actually actively hinder relevant matches unless you pay them and keep paying them. Places like Reddit are hit and miss, heavily dependent on the time you post, and you not only have to risk your privacy to find good matches, you have to gain the approval of everyone else who could remove your post from visibility.
I miss IRC, but of course even that was far from perfect, and had its own risks.
This isn't just about dating, but finding friends with similar interests at all.
With 8 billion people on the planet and all the connectivity, you'd think this would be easier for everyone by now.