POF has from 450,000 (during a weekday afternoon) to 1.5 million users (during the evening on Sunday) online at any one time. If you are new to POF, you are not alone, about 70,000 new users sign up every day. POF.com gets about 2.4 billion page views every month, and every day on average 4 million members logging in resulting in about 10 million conversations.
POF got its start in 2003 (as Plenty of Fish) and is now one of the world's largest free dating services with more than 150,000,000 members since it started.