My main problem I have with Facebook pages for designers is what is the incentive for people to 'like' your page. If it is just another place to post links to blog posts I don't see the incentive. Maybe it's just the fact that I like to keep Facebook for personal and use twitter and linked in more for business, which I think is quite common.
I've got retail clients that have great facebook pages as customers often post questions and get answers straight from the company, or other users. They offer facebook exclusive discounts, polls for new product development etc. It works because it builds a sense of community amongst the customers and gets them involved.
I don't know what the answer is for designers but I think if you are going to make a facebook page reach beyond your inners friends/ family you need to think about why someone should bother.