Take a look at this post from Econsultancy that considers who is doing it well (and a few flops). In particular the example of how Etsy is using Pinterest for its business shows how you can get it right.
What stands out for me is how important it is for businesses to recognise and work within the excepted (unwritten) rules of of an individual platform or network to get the real benefit.
The reason for this success is the following of a simple formula: selling is not forced on the user, it’s offered as a possible conclusion of social engagement.
This stands out as a great tip on how to make your social channels a real success and to get a return on the marketing budget invested.