For a B2B campaign, what is the best social media platform for advertising?

Social Media is good at consumer level advertising, for B2B better go for Newspapers/Magazines.

Being a business-centric channel, LinkedIn was always at the top of B2B social media activity. You may also look forward for Facebook and Twitter but the only drawback with them is that they are also considered to be among blocked websites in office premises.

It depends on the industry generally. LinkedIn can be useful and no matter whether it is B2B or B2C - Facebook and Twitter.