What's the base rate on lackluster social skills? Based on the popularity of self-help books and seminars aimed at improving social skills, I'm led to believe that social butterflies aren't all that common among the general population either.

Most people pick up a huge amount of tacit social knowledge as children and adolescents, through very frequent interaction with many peers. This is often not true of intellectually gifted people, who usually grew up in relative isolation on account of lack of peers who shared their interest.

Curious use of the singular "interest". Somehow I don't think intelligence is the real issue here. Rather, it strikes me as a consequence of diverging interests. Let's take youth/high school sports teams. There are the so-called dumb jocks and then there are athletic geniuses (for example, Alan Turing was an extremely good runner). You could easily end up with a skilled team featuring a large gap in IQ scores. The endpoints of this gap would have overlapping interests despite the intelligence difference. It's the people who focus their attention on a narrow range of topics outside the mainstream who are likely to have the most trouble.