A 52 page report from ComScore, from 2010 shows PlentyofFish (POF) is for casual dating (short term relationships).
IAC Personals (Match, Chemistry, OKCupid, Meetic, ParPerfeito, MeeticAffinity, SinglesNet, People's Media Communities, DateHookUp and others) is in the range of USD750 million per year.
Revenue of eHarmony' sites (USA, Canada, Australia, UK and Brazil) is in the range of USD250 million per year.
Annual revenue of IAC Personals + Revenue of eHarmony' sites == USD1,000 Million == USD1B
Annual revenue of PlentyOfFish Media's sites (PlentyOfFish, eVow and several dating sites powered by a white label dating company) is in the range of USD40 million.
USD40million / USD1,000 Million == 4% of market (in revenue).
PlentyOfFish has the traffic but others receive the money.
Free/freemium online dating sites like PlentyOfFish and OKCupid are only marketing tools useful to send prospective customers to paid online dating sites like Chemistry and eHarmony.
When free users got tired of free sites, they migrate to a paid one.
If the owners of PlentyOfFish can not innovate in the Online Dating Industry, the best strategy for them is to sell PlentyOfFish.