^I would agree with most of the above comments...
but also add that sometimes introverts do feel the need to socialize out of etiquette. But maybe that's me being an SO instinctual variant. I just bring my Se to the forefront in those situations and can be quite a goofy, yet charming young lady. Though introverted, I have no problem engaging in conversation with others. However, it's less because I feel as though I want to engage, and more as though I feel the need to engage... like it's just the nice, friendly thing to do. Perhaps that's where my dominant Fi lead me. IDK.
With shy extroverts, I think once engaged fail to disengage easily. Where as with myself, I can engage easily, but fail to keep it up for an extended period. I have many many acquaintances because I'm always friendly. I have few close friends, however, because honestly, I'm just not that sociable.