"Social selling" gets all the headlines these days. But if I were asked to train salespeople, I would place any of the following seven things before social selling.
How to cold call and book appointments. There isn't anything higher on this list because cold calling is what would improve most salespeople's results faster than anything else.
How to overcome objections (or resolve concerns). No matter how good you are, without the language and rationale to deal with objections, you won't be able to create or win any opportunities.
How to differentiate themselves and their companies. I've never asked a salesperson what makes their company different and gotten a compelling response, even when their leadership team believes they know what those differentiators are.
How to leverage the buying cycle. Salespeople would benefit more from knowing how to serve their dream clients through the stages of buying than from learning how to market on Twitter.
How to recognize an opportunity. Unless and until your dream client agrees to pursue change with you, you do not have an opportunity.
Why they need to follow a process. Most companies don't follow a process, and neither do their salespeople. I'd teach them why they should follow one and how it will help them win.