I was using social media for past many years for my business promotion. At my experience Facebook and Linkedin both give me best output of my business, I got lots of traffic and inquiries.
There are so many people think that facebook is the top social media site but it is not. Twitter is the most popular social media site with Google Page rank 10 and the same facebook has got Rank 09 on Google. So, you know that who is the boss of social media.
In Italy facebook is by far the most widespread and most used, I can count on the fingers of my hand people I know under 50 that aren't at least subscribed, and most of them at least use it regularly.
I guess it depends from the country. In Italy for example linkedin for example is still not widely used (even though there are still many people subscribed even here). Same for Google +, not used much here.
Facebook is the most favored web page and there also other sites such as tweets, linkedin, Pinterest, Search engines but each web page has its own advantages but mostly people use Facebook as their online community web page.
There are many social media apps that are vital to build business but out of them the best one are s follows:
Facebook
Twitter
Google Plus
LinkedIn
Pinterest