Facebook have over 80 the last time I checked.
We have 3.
I always turn them all off (on FB, Twitter and elsewhere) because I'm on site enough to know what's going on, but some people find them useful.
All we have currently are:
Send me an email alert when I receive a message
Send an email when a new friends request is received
Send me an email when a comment is made on my stories or profile page
Authors might like to know when they receive a comment on their story for example.
Does anyone have any suggestions for additional notifications?