There are definitely fewer people than e.g. Tinder, but they tend to be a lot more real vs spammers / scammers / engineered profiles designed to keep you in.
Most Tinder matches were bots, most OkC matches were actual people. Of course this is with a sample size of 1, in Germany. Your mileage may vary.
Is that grammatically correct? It appears to be, but wow does it ring strangely in my ear.
Maybe it is a bit ambiguous, although from the context it should be reasonably clear that I was with my (now-ex) girlfriend for that period of time, and not with OkC itself :)
I don't think what you said is technically correct. "With whom I was" calls for something (a verb or phrase) to be tacked onto the end, but it would still be awkward. For example, "with whom I was dating for three years."
Actually, I'm pretty certain you're supposed to use "whom" there, not "who".