Hi I've sure that Google will give preference to their own social channel G+. I've heard from bloggers that using G+ helps in a great way. Not sure about FB though.
You can check this on whatever tool you are using, for example - Google analytics. Log in to your analytics. You can check there for social site resources. See what social sites your traffic are mostly coming from. It will give you an idea then to answer your question above.
Hi, all the social media platforms are good for promotion because they all have the similar popularity but if you are wanted to promote a E-commerce site I would like to suggest the Pinterest.