LinkedIn can be a great place to announce meetings or other events that are professional in nature. You may want to make sure your regular contacts find out, but you also want to attract new interested parties. Yes, Facebook has more members, but many of them are engrossed in Farmvillle II and may not be able to participate.
I prepared a little how-to video. Please feel free to critique it below. I am always looking for things I can teach my colleagues about using interactive web tools!