1) Be social: Wherever two or more of your customers are gathered, so should your dealership be there also (to paraphrase), and that applies to social media. And, if you're not there personally yourself, too, get on Facebook NOW--having a business page and not being on Facebook yourself is like buying a newspaper ad when you can't read.
2) Start smart social: Launch your social media business pages, etc. with a plan, or don't do it at all. If you can't monitor it, don't have it; if you can't add interesting content on a good schedule, don't start it.
3) Keep it social: Your Mom wouldn't care to see your inventory stapled to her walls--at her house OR on her Facebook news feed--so don't think your customers will like that on Facebook, either. They'd all like, however, to hear about your customer's new truck that trailered their boat to a great lake trip. Act otherwise and your page will be ignored by your customers--and you won't even know it.