Read the fine print -- it's top sites by pageviews, which is a very different (and less meaningful) metric than top sites by unique visitors.

That same Alexa data you quote has POF at 168 in the US and 466 worldwide, with 21+ average pageviews/visit (Compete has them closer to 30). According to that same data, Flickr has ~9 PVs/visit, and CNN has ~3.5.

It's not inconceivable that with a few differences in the data between Hitwise and Alexa, you could come up with POF in the top 20. It just shows you how meaningless pageviews are as a metric of success or engagement.