Finns (disclaimer: I am one) are probably the archetypal introverts.

When a stranger on the street smiles at you, you assume that:

a. he is drunk

b. he is insane

c. he is American

d. he is all of the above

-- You know you've been too long in Finland when...

Alas no, there is no social pressure to be introverted here in the UK, at least not in my generation and younger. We have the same celebrity culture which idolises the loud and outgoing as does America.

Middle class, middle aged British people are a bit more reserved than the younger generation.

I live in Ireland, and the social pressure in favor of extraversion is at least as great as in the United States.

I apologize for misreading your comment. When you wrote:

culture affects how social people are expected to be.

and

extroversion is much more compulsory in the US than in a lot of other places.

I had thought you were suggesting that cultural pressure would influence people to become extroverts (or introverts as the case may be in some cultures). Actually, I would guess that the dominant social culture would cause both these things: 1) some people, probably influenced from an early age, would be more likely to become extroverts in an extrovert-dominated culture (or vice versa); and 2) some people would feel unhappy because they didn't fit into the dominant culture.