If you’re a subscriber to a pay dating site, you are an important (though unwitting) part of that site’s customer acquisition team. Of course, they don’t want to show you too many ghosts, because you’ll get frustrated and quit, but that doesn’t change the fact that they’re relying on you your messages are their marketing materials to reach out to non-payers and convince them, by way of your charming, heartfelt messages, to pull out their credit cards. If only a tiny fraction of your message gets a response, hey, that’s okay, you’re working for free. Wait a second…you’re paying them.
That’s probably the same reason why you see the “hey, you haven’t posted on so and so’s wall in a long time” messages on Facebook.