PlentyOfFish is a legitimate service to utilize to meet other people. At any given time, there are at least 250,000 members online. The number is sometimes much higher depending on the time of day.
PlentyOfFish is a free service with some paid upgrades available. Basic user membership allows good functionality with the ability to exchange messages with other users. Unlike some of the other services, a user does not need to upgrade to interact with other members.
A feature of PlentyOfFish is that it offers various tests to help members identify their personality types. The site then suggests possible matches based on the results of the tests.