Advantages:
You generally have a larger quantity of friends.
Hardly ever have to worry about not finding a partner for a class project.
Have a much greater chance of getting a boyfriend/girlfriend.
Most teachers seem to like you better.
You learn to be more comfortable around people.
Always have someone to talk to/hang out with when your bored.
For some reason, regular/unpopular people tend to look up to you... even if its only secretly.
Don't get picked on.
Someone always has your back.
Self-esteem boosting.. usually.
Disadvantages:
The quality of your friends declines as the quantity rises.
Have a tendency to become vain and/or egotistical and/or shallow.
More likely to get into fights (cat-fight, anyone?)
More likely to get pregnant or do drugs.. seriously.
Generally pick up some rude habits.
Difficult to find alone time.
Always feeling the pressure of having to keep up with all the new fads.
Could spark jealously in less-popular people.