Most of them were acquired by match.com, to even avoid the chance of any competition. How regulators allow this monopoly should be a question for criminal court (similar to many other industries, like letting facebook purchase instagram and whatsapp many years ago)
The chicken to egg metaphore is perfect, though they could be forced to change with strong regulation. Monopolies should be either broken up to allow true competition, or should only be allowed to operate as thightly regulated utilities (like electricity distribution networks).