Every great salesman I ever worked with back in the "olden days" kept 2 boxes of cards under or in their desk. One was filled with greeting cards, the other was a tickler file of customer's dates: birthdays, wedding anniversaries, the anniversary of their last car purchase, etc.
No one ever expected the card they sent on a Monday to sell them a car on Saturday. They did expect to stay top-of-mind with their customers, maybe get a referral or two and have a better than average shot in a year, 3 years or 5 years when their customer was ready for a new vehicle.
I'd submit that Social Media is the equivalent of those greeting cards. The ROI won't be measured in days, weeks or months, but rather in years. Is it worth the time? Every dealer has to be the judge of that for themselves.
But I'll repeat, every great salesman, back in the day, kept in touch, developed an on-going relationship, built trust and a referral business. Even if the medium is different today, why can't we do the same thing?