Here's a good overview of the ratings situation in late night TV. Of special note is the age of the audience that each host is attracting. They probably all skew older than you'd expect.
These numbers bode well for Mr. Fallon and I believe he's doing much better than Conan O'Brien was when he moved into the 11:35 time slot. The question is whether Jay's viewers will stick with him or if they'll go away as rapidly as they did last time. If I had to guess — and that's all this is: a guess — I'd venture that an awful lot won't stay with Fallon. A few will go to Dave and fewer will go to the other Jimmy…and we'll look back and say that when Jay left, he took a large chunk of the late night audience with him.
Also surprising to me is how so many folks who watch Jon Stewart don't stick around for Mr. Colbert…but then what we're looking at here are the averages for an entire episode of each show. When you see that Letterman has 2.95M viewers and Ferguson has 1.48M, your first thought is that Craig is losing half of Dave's audience. But Dave doesn't have 2.95M people watching the last minutes of his show. He has more than that when he starts and less when he ends as more and more people go to bed or change channels. To really assess Fallon's strength, you'd have to have the numbers for Leno's last 15 minutes and lay them alongside the numbers for Fallon's first fifteen. Maybe NBC's decision would make more sense if we saw those stats. And maybe I'd change my guess if we did.