Since I'm a sophomore in college, I'll talk about university involvement.
In that case, I'm too involved sometimes. In fact, I had to dial it back this year. On top of taking the maximum 18 credit hours last semester, I was: on a Student Senate committee; on my hall's executive council; a chair position on our Student Activities board; involved with a Residence Hall Association committee; and a part of our Quiz Bowl team and Democrats group.
That was all fine and dandy, and I made Dean's List and had a social life and even came back to ATRL, but this year is different. My classes will be a breeze, but I've been selected as the Student Senate Historian and the Residence Hall Association Historian for the year, with the latter coming with my own committee to lead. I was rejected from a second year on the exec board of SAB for reasons undisclosed to me, but the president of the board invited me to apply for committee positions and a spot on the Drake Relays Executive Board (which is actually in charge of all student events for the week of one of the biggest track & field gatherings in the nation). I'm also considering applying to be a part of my particular college's ambassador program and on a Student Senate committee (even though I've already got a non-voting full position on the primary board).
I don't do social fraternities or professional ones; I may join the latter for business prospects soon but probably not.
I find most school affiliated functions to be quite lame, in addition to being unsuitable for the introvert who just wants some time alone after a long day surrounded by others. So, I basically do the least amount of extracurriculars I can while still getting by.