My guess is that you don't have a Facebook account or you rarely use it, because the issue of Facebook goes well beyond "crack" or such.
A.) Everybody at your university has Facebook.
B.) You need to communicate with a group, and by "communicate", I mean time, date, activities, who is doing what in those activities, what is required for those activities (eg; clothing? tools?), everyone's office hours, and a plethora of other information.
C.) Everybody has very random schedules, and at least a few live 30 minutes out of town
Anybody who even dares to suggest email for this is utterly clueless. Also, anyone who suggests that "everyone just use this-other-system instead" has clearly never tried to get a large group of every day people (particularly college students, none of which are technology majors of any sort), who are in turn tied to other people who are tied to the system who are tied to other people, to begin using something new.
Additionally you have the international students who are nearly impossible to keep in contact with, especially through any other means. Typically they'll have their own system (eg; Cyworld), but those systems will require a national identification number that only citizens of that country have. Therefore you can not join those systems and thus you use Facebook.
Lastly, the friends who you have, and who all have random schedules due to classes that take place all over the day-by-day calendar in addition to jobs, sports, and other activities all have Facebook. Ergo you all continue to use Facebook. Again, just try to get these people - who know people other than you who are all similarly tied to Facebook - to use anything other than Facebook.
If someone still can't understand all of the above and STILL says "will I don't see why you don't just close your FB account & use this-whatever instead" then that someone has clearly never socialized and they probably should not comment on a subject which they have less than zero understanding of.
It's not about YOU, it's about EVERYONE, and that's why you can check in but you can never leave.