I don't think anyone can say whether or not the FBI dropped the ball. It will take a Congressional inquiry for sure. Most likely at some point Obama will want to know more about how the FBI handled it, so will the FBI director.
He visited Russia, they thought it was suspicious, he returned to the USA and Russia thought enough of him to warn the FBI about him. Russia probably had nothing more on him as he only traveled there once. After that it would have to be American intell watching him. Houghtam we aren't talking about drug deals so what does that have to do with the FBI tracking terrorists they were tipped on to by another countries intell? I mean trying to equate that with joe blow American telling the police the neighbor could be a drug dealer is a huge reach even for you. Well maybe not for you.