James Franco is, to put it diplomatically, a bit of an odd duck. Sure, he's a very talented guy -- and one who has never balked at using his talents across a wide range of professions -- but he's also kind of a weirdo in plenty of situations, especially as they apply to social media. And David Letterman is not having it.

Franco hit 'The Late Show' last night to chat about a wide range of topics, but Letterman was fixated on one thing -- that icky underwear pic that Franco posted and deleted last week. The late-night host skillfully got to the bottom of Franco's obsession with the social media channel (and we don't buy for a minute that Dave calls it "the Instagram"), and he might have finally figured out just what about Instagram calls to Franco. (Hint: it has something to do with attention.)

As a special bonus, Letterman and Franco also chatted about the actor's current Broadway run in 'Of Mice and Men,' a super serious outing that somehow doesn't keep Franco safe from hooting and hollering at teenage girls. Woo woo!

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