For product based industries:
For service based industries:
- Google +
As i stated in anothe answer in 2011 we saw social media account for apx 25% of sales for most business that actively used it interact with their customers.
-> Personal and professional then, Twitter, Facebook, LinkedIn...
-> Regional, i.e. Chinese market then, RenRen.
-> Keep an eye on Diaspora, too. It's the new development based on P2P social networks; this means, you will be able to host the software yourself so your data will be in your server.