I do not use one currently because I am not soliciting new students right now. I am fully aware of the advantages of Facebook, Twitter, blogging, etc. as well as optimizing your website for search engines. These are all tools any school seeking to attract new students should be using - old advertising methods like the printed Yellow Pages are probably close to useless at this point.
I am planning to start publishing some videos of the type of training my dojo offers on Youtube. This will be more for informational purposes than marketing, to let prospects know exactly what being a member of my school is like and what type of training they would expect to learn.
Well, considering that the yellow pages will be in every house in your zip code compared to the international public you get on the net...but then again facebook vs paid add...
one more point: I have facebook and myspace but I never check it out, and definitely not updating. So if you are not 100% behind the idea of having one and just doing it because it's what people do, get somebody who is into it (like wives and kids etc, nt saying guys don't get into it) to keep things going.
I have yet to upload my first picture to facebook...or myspace for that matter, double-triple checking that only approved friends get to see them.