The problem with free dating sites is that they have to be fucking enormous to make any money. OkC is an amazing product with a huge user base and guess what? They had a tiny office and weren't making much money, then they were bought by (shock) a dating site with a sustainable business. IAC/Match sucks but they understand something the OkC guys don't—how to make money from their product.
And selling a business with $6 million in investment for $50 million is likely to be lucrative enough, anyway (and you'd hope that they were turning some profits to justify the $50 million.)
From what I can tell, the article you linked says that PoF is making $15mm in revenue.
If OkC were making 15mm in revenues why in the world would they sell for $50mm? That just doesn't add up. Their overhead outside of bandwidth costs wasn't that high, they employ 10-15 (I think) employees and have a small office in Chelsea or sommat. Their profits would be enormous if they were generating anything near that.
