I'm Lutheran and so are about 40% of the people I know. I'm kind of suprised whenever people outside the Midwest think Lutherans are pretty rare, I keep forgetting that almost all of us in the US are here.
Another thing is that plenty of people outside the Midwest think all Lutherans are ultra-conservative, which is largely true outside of the Midwest, Scandinavia and parts of Germany, but it's definately not true of the majority.
They were the first Lutherans that I knew about, I might have known others. And, as a Catholic, most Protestant sects are not that different from one another. I just thought Lutherans were a mainstream branch of Protestantism.
By the way I felt the same way growing up, I used to think we were a majority Catholic country, because there are so many around here. And I knew a lot of Jewish kids, I went to two Bar Mitzvahs.