Social media lead capture / lead nurturing

We working with a marketing company no contract, just a gentlemans agreement but they bring us high value leads straight from what it seems Twitter and Linkedin etc..

Maybe £20k in the last few months worth of development. We are a mobile application development agency.

Its not all B2B its mobile apps from startups who have raised investment funding, private projects and some business.

They are the middle man in a sense, they find the lead and talk to them, ask us for a price we give them pretty much a trade price, they add whatever they want on to the price and close the deal so its an a stage to be invoiced, we invoice them and they invoice the client.

But it is working well, I am wondering if there are any of those social media marketing people here?

Cheers.