My mono (K) and I actually met on eHarmony. I had gotten out of a relationship where I was trying hard to be a good little mono and signed up with some extended deal there. After signing up I realized that I wasn't willing to try at being mono anymore and came to full identify as polly.
I'm pretty sure Dr. Clark (eHarmony guy) would have booted me hurry-up-quick if their gears and whistles flagged me as poly. In any case, a little before that expired, K and I were connected. I would actually advise against eHarmony for polys (though fine for monos) because it doesn't suit the lifestyle. I was just lucky that my transition to no longer kidding myself (about five years ago) was timed in a way that I wound up meeting K, and I'm extra lucky that part of her awesomeness includes being poly-friendly.
Partners since then have been met through getting out, interacting in other activities, and being bold enough to bring it up. Or they're beloveds who I have been separated from by geography but not internally.