James Franco's been kind of quiet since he sent out a sterile, half-naked bathroom mirror selfie to his millions of Instagram followers last week.

Of course, the topic came up on Letterman tonight and Franco sort of explained things.

"To me it's just a fun thing, it's something I don't put a ton of thought into, but it gets a lot of attention," Franco said, noting that he has "a lot" of Instagram followers (more than two million).

He later added, "It's not like I'm putting that on billboards. Ostensibly Instagram is for my fans. But now all the bloggers are on there so they'll just take it and use whatever they want."