Compete.com has a great post comparing Unique Visitors VS Actual member logins to dating sites and show the top 10 rankings for each. Clearly ranking by member logins is the most important factor because only members can talk to each other and the more members come back the more relationships are formed.
Of the top 5 dating sites listed in terms of member visits only Plentyoffish has a market presense in Canada. Canada is the primary market of Plentyoffish.com with way over 10 million monthly logins here. This easily makes Plentyoffish.com the Largest dating site in North America. If we throw in UK and Australia data Plentyoffish is easily the largest dating service in the english speaking world beating Match.com in the 2 most important metrics which are member visits and total relationships formed per year.
This causes me grief, my entire marketing to date has been about Plentyoffish is the under dog and you should use Plentyoffish because its free!. But now that plentyoffish is transitioning to market leader in many markets its about expanding the dating industry and not stealing users from other sites. This tends to be very expensive both in terms of People needed and $$$