Personally, I would still think social media optimization would be another channel for IM purpose but not be included in SEO though Google and Bing both include the social search ranking in their SERP results.
all the best,
I got your point, but think in that way not every person on the web are not same and so link posted by different person will have different link values. So if you are on top of your social media your social signal will be stronger and will definitely rank higher on SERP as site maintained will get a proper share of your reputation,
Thats why i am thinking the impact of social signal on SEO will be significant.