Remember that old saying, "Opposites Attract?" My husband and I are a pretty good example of that being true; I am an 'In', and he is an 'Out'. By 'In', I mean Introvert, which leaves 'Out' to mean Extrovert. People use those words all the time, but I'm not sure they truly understand the meanings of both. In particular, the members of one group do not understand the needs of the other group, and vice versa.
Hubby is an extrovert. He loves to be around other people (with very few exceptions), and the more the merrier. If they have something in common (and that can be as small as they are both wearing black t-shirts that day), he is happy to explore their individual universes via conversation for hours! He is the type of person who, even after he acknowledges that it's time to leave, will continue to converse for another hour... or three. He leaves these convers-athons full of energy, with new tidbits of information to sort through and cubby-hole away, until he can find another conversation where he can pull them out and add to the mix.
I, on the other hand, am an introvert. Yes, I guess I'm shy, but introvert is more than that. Yes, I often am a wallflower, but does that really tell you anything about me? No, I am not anti-social; I like people just fine, in small doses, meaning one (okay, maybe two) at a time. Put me in a crowd - like a large convention - and I am overwhelmed. I pull inside myself and probably put barriers up that tell others, "Don't bother me." I might work on my laptop or pull out something to read.
When we go to big conventions, like Megacon or a Star Wars Celebrations, Hubby gets frustrated because I don't want to go to the dealer's room. Actually, I'd love to go see all the merchandise, a lot of which I don't see at smaller conventions, but a football field-sized room packed so tightly with people you frequently can't move more than a step, with a sound level that keeps you from thinking, with colorful displays that surround and tower over the crowd - that's not my idea of fun. It is my idea of confusion, and borders on panic. It's a situation where I have no control, and coping with it saps all my energy.
In short, extroverts find it energizing to be with other people, and are probably bored when they are alone. Introverts find their energy when they are alone, and are drained when they must deal with crowds. So if you have an introvert friend, don't think you are doing them a favor by dragging them out to a club. They will probably appreciate a movie a lot more.