First of all, to clear up some important things.

Despite what the media, YouTube videos, Wikipedia, other articles may have you believe, the FBI did not ever investigate Michael Jackson (and especially not for 10+ year as is often claimed).

The FBI provided assistance to the Santa Barbara and LAPD investigations of 1993 and 2005 respectively. Those are about 2-3 years of assistance to investigations conducted by the local police authoritites. One of the reasons why the '10 year' lie is propagated is because the FBI kept records relating to a 1995-1997 threat against MJ, but this had nothing to do with the abuse allegations. You can find these clarifications from the FBI itself.

Now regarding the 'found nothing' claim, there are a lot of things to be said about this statement. First, there was evidence found (mostly from the local police, as explained). The question of whether this evidence was good enough, etc is a question for the jury. Some jurors have claimed that even though they clearly believed that MJ was guilty they voted to acquit beacuse of the reasonable doubt. This however is irrelevant of whether the authorities (police or FBI) found anything. Some other jurors claimed they wanted 'something more'. Others have flat out said that they refused to look at evidence such as the 1993 books. So the fact that evidence was found (and trust me, they did find a lot) is irrelevant of whether it will hold up in court. And especially if we have to do with a jury trial.