Fallon is just... is just terrible.

I know Fallon loves doing the game stuff, but he really is one of the worst choices to promote it.
The market is split into two main groups. People who know what the 3DS is, and the people that don't.

The people who do know about the 3DS already made the decision to buy one or not. The people who don't know about 3DS most likely watch the more popular Leno, Letterman, Conan and then go to bed before Fallon. So marketing on Jimmy Fallon isn't exactly a hard ball move.

But its not a dumb move to be on Fallon altogether. Even if its on a late night show no one in the target demo-graph ever watches; Nintendo doesn't have to pay anything to show off a quick 3-4 minute demo of what their product is all about. Its free publicity. Fallon show gets paid by sponsors from ad-time, and Nintendo gets to have a full 5 minutes of glory on NBC showing some stuff off that a 15-30 second, $1,000,000 ad-time commercial can't show.

All 3DS stuff aside, Jimmy Fallon isn't all that bad.
Nightly Talk shows follow a regime. Monologue, then Skit, Main Guest, Sub-guest, then Music Guest. Fallon's ok with Guests, but he makes for a terrible host with monologue and skit times. He sucks at them, and he's terribly awkward. And that hasn't changed since he started out.

Coco's still the best, but he doesn't do as well as Leno because Conan's nature alienates the elderly audience.