I will give my "two cents" as a potential customer. Whenever I am searching for quality detailers in my area. The first thing I do is a google search. If I find a detailer I'm intrigued by, I'll try and see if they have a website or a facebook page. Some detailers I know purchase ad-words to get their site ahead of others. Additionally, have your facebook page be able to be searched by Google as well.
Remember, just because people don't post, doesn't mean they don't look at your pictures. You'd be surprised how many people will look at your page to see pictures or rates.
Facebook is a good tool if used effectively. It's free, and like Bill said, you can use it for incentives. You should also get as many friends and family to "like" your pages as well.
I always tell my detail friends to use facebook and craigslist as much as possible. If only to get your "name" and services out there.