signed,I have several accounts.
One is for business/work contacts, they can add me if they want, I don't care, I rarely access the profile but it gets them off my case when they ask me if I have a facebook account. I hope to delete this account at some point, in the meantime it's the one I would make look respectable and have viewable by potential employers when job hunting.
One is for people I know irl and would like to stay in contact with but do not see on a regular basis, it can be quite useful to know what is going on, I log in every once in a while to keep abreast of things but I never post anything on my wall. If someone tags me in a photo the tag will be removed, I purposely don't want people to know what's happening with me.
One is for online people (and citiville )
All in all I do not use it as a social networking tool very much, it lets me know what's happening with other people who seem to use fb as their only way of telling people what's happening and as such is quite useful, but it doesn't tell them what's going on with me.
If your professors are using it to communicate with students you would be putting yourself at a disadvantage by not setting up an account.
4evah yours entropie