Essentially you will be taken by surprise, and forced to buy the premium if you want to keep messaging people. It may say you have 4 messages left, but all of the sudden you will have zero after sending one and getting another back... I think this is a dirty trick, and while I do find some people worth talking to, this is a deceitful trick attempting to get your money.
I'll give it 3 stars because if you find someone, then it is worth $10.