Not on Myspace! It caters more to a younger crowd. Everyone is going to Facebook. I would advertise there before Myspace. You can create events for free on facebook, post links all kinds of good stuff. If you don't have an account there yet that what I would do. You can look me up Mark. James Flournoy Forest Park, Ga.
I agree Facebook is more for business networking while Myspace is a great site, it caters more to the social aspects then business, even though Tom (started myspace) is making a killing on the adverts for myspace
There have been many studies on the ineffectiveness of Facebook ads. They can be cheaper than Google Adwords, but the conversion is downright awful for most advertisers.
With Facebook I think your best approach is to follow jamesflou's advice. Create a page for your agency, encourage your clients to join your group, etc. Be creative.