Usually the split here depends on how a person "recharges" when they need to. This is a continuum, so it's not a one thing or the other. You can be anywhere from one extreme to the other, and in different spots depending on your mood or situation.
Generally speaking, introverts want/need to go be alone or with one other person to recharge, while extroverts get a bit stir crazy when alone for very long and really get revved up by being out with other people.
There are many people who are more introverted, but can be the life of the party in a fun situation or on stage. They just need their "alone time" to recharge later. The more extroverted types need little or no alone time to recharge.
(yes they call me doctor at work

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I'd like to add on to this, seeing as how I regularly test INTJ (Introverted, iNtuitive, Thinking, Judging-- although I occasionally test INTP) on the Meyers-Briggs test. I would like to dispel the myth that introverts want nothing to do with people and are socially-awkward. It has nothing to do with that.
As SR505Bassman touched-upon, it is more about what stimulates you. Extroverts, generally find social interaction stimulating. Introverts, on the other hand, generally find social interaction (at least with a high number of people, or extended periods of time) mentally and emotionally draining.
I can relate to this, because I'm a bartender. I have to deal with people every second for 9 hours a day. If someone brings up a topic I have a great interest in, like music, the Pittsburgh Steelers, or whisky, then I can hold conversation at great length, so long as the discussion stimulates me on some intellectual level. If people insist on talking to me about things I really couldn't give a shite about, then I find myself struggling to do so much as nod and smile at the right intervals, then look for some sort of work to pull me away from said discussion.
Browsing forums, taking in other peoples' experiences and contributing my own, is something I find stimulating. The same goes for reading a book, doing some math problems (I'm studying to be an engineer), or jotting down some ideas for a future project bass on a tablet of paper (I've got a lot of ideas, but very little funding!).
For those reasons, I'm happy being the bassist. I'm happy not being on the spotlight. The only downside of that is when the gig is over, both the frontman and the guitardist(s) get the first pick of the girls!