Generally speaking, large companies tend to suck at executing on anything but their core products. Other startups are a far greater threat, simply because they tend to move faster and with greater focus, usually with a much higher-quality product.

In this case though, I'd say the threat is about equal since eHarmony has a massive amount of users. Keep in mind that many startups looking to foster a two-sided market fail because they can't reach the minimum threshold of initial growth required for their product to work as intended.