Really, this is too easy to game to worry the cynics - the site owners could log in and continue a few conversations for a bit to make the bad publicity go away.

If I was running this kind of operation I'd probably ensure some conversations continued for a couple of messages after users paid up, just to make sure they didn't cancel or file chargebacks. If I assume men preferred their hot women to pass the Turing test I could probably outsource the flirting to someone in the Philippines at $5 per hundred messages.