I find time and time again that fun and unique news gets more attention than any coupon or contest. If you're doing something fun like having a BBQ, petting zoo, treasure hunts like we do, even starting a conversation about a controversial movie that just came out, you'll get more attention and more sharing.
Better to actually be interesting than to offer a gimmick and come off as salesy.
Anything that is about the customers. Make it about them, not yourself. If there is a big rivalry between two local sports teams, ask them who's going to win or who they are rooting for. Get a competition going.
Ask them what brand of coffee they drink. Things that are varied with everyone usually peeks their interest because they can get a glimpse into other people's lives too.
And don't forget the occasional Thank You for being a Fan - Fantastic deal.
I think it is all about being "social" Not so much pitching, just sharing information that will help others down there path and supporting others. Now if your comments are slanted toward your company or the company you work for... So be it. Than it is professional advice.
Humanize your dealership!!!! The goal of any good social media strategy should be to set your dealership apart from your competition in ways other than price, etc. It is unnatural for people to WANT to be friends with a car dealership but you have to make sure your community understands it is just a business with a bunch of good people that also live in the community and they don't all of a sudden become mean people when they walk through the doors of the car dealership.