On a cost per lead basis, LinkedIn is by far the most valuable in my experience (b2b software sales), especially compared to display advertising or even Google

On a cost per lead basis, LinkedIn is by far the most valuable in my experience (b2b software sales), especially compared to display advertising or even Google Ad Words. Twitter Ads (specifically promoted tweets) were also a great source when it came to clicks. Depending on your business model, some sort of display ad retargeting is also key (e.g retargeter).