At the time (mind you, this was 10 years ago) eHarmony had a "tell": paid-member profiles were rendered subtly differently than free ones. I got fed up with using the site and decided I was just going to game it and see what I could learn, so I started taking meticulous notes on what I saw. I took every match that was sent, I kept notes on how I replied, and eventually some patterns emerged. Eventually had it confirmed when a person I was matched with, and went on a date with, decided to let her membership lapse and suddenly her profile was rendered differently.
eHarmony's system only let you message people from a daily, "curated" list of matches. On any given day, you'd get 6 or 7 profiles on your email to review and decide to message or not. I'm sure you just did the math, 10% of 7 is less than one. Less than one viable connection a day.
Once I got past that "90% of these matches are dead ends" hurdle, it turned out about 50% of the people who were capable of replying actually did. I don't remember the exact breakdown, but basically there were a number of questionnaires you why through before getting through to love messages and asking people out. It was essentially a 50/50 filter at each step. IIRC, it was something like a half a percent chance that any profile eHarmony sent you would end up in an actual date.
I suppose dating sites figure, if someone finds their soul mate, they won't need to pay for a membership anymore, so don't actually match anyone, just strong them along for as long as possible.