Manti Te'o knew his girlfriend was fake and used this stunt to try to gain notoriety and popularity. Given the popularity of a college football player on campus, there would be no need for Manti to go online to find a random girlfriend. He did this as a publicity stunt to attract attention.
Of course, the only person who may know the truth about this is Manti himself. If he didn't think she was fake at first, certainly he should have known whether she was real or not when he went public with the situation. I also find it hard to believe a star college athlete would need to look for love anonymously over the Internet.
It's a sad day when an athlete has to scam the public into liking him for more than his athletic ability. Some athletes do have some sad situations in their lives and it is really the fault of the press when they present these stories to the public. It gives the press more to talk about and they can influence how people feel about a player quite easily. If Manti Te'o was aware that the story was fake that is a very sad thing for him to have done. I think probably at first he was the one being scammed and by the time he realized it the story already had a life of its own so he didn't know how to get out of it.
I dont think he did. Many people fall victim to fake boyfriends/girlfriends, a good documentation of this is the show "Catfish." He is just another person looking for love, but looked for it in the wrong place. He has nothing to gain by going through all this, in reality he had more to lose because of people accusing him of knowing everything was a scam.