I guess the whole notion of 'social learning' is a bit flawed. To collaborate in the scope of a course isn't new. That happened even in the days when we had no tech and did just classroom training.
The key with 'social' is the network effect, serendipity and the fact that you have more eyeballs looking.
I'm also unsure if I can ever quantify in dollars the return on investment. I could potentially outline the value itself.
In that I have highlighted community engagement as a metric (contributing vs active members), no. of answered questions is an interesting one. It'd be useful to look at the different roles in your company and see how engaged they are on the platform. Using net promoter scores is a good way to measure satisfaction and I'd love to do this for my own community at some point. I'd love to find a way to quantify serendipity - how many people accidentally find content that's useful. Not sure if I know how to do this yet.