There is no middle ground.
I've heard of a few schools around here that have facebook/twitter and see it as a waste of time as no-one follows it.
And others that think it's the best thing ever, because a large majority of the parents have added them and are using it.
Send out a newsletter or 2 [or 3... we all know how easily children lose letters.] telling people about it, make the account and see how it goes.
If it isn't sucessful, at least you can say you tried.