It seems implausible to turn this into a business. So, you have this seed group per network and, what, charge some amount to reach this group on the assumption that they will care enough to propagate the message? Seems that quality and relevance of content are still the overriding factors.

Also, wouldn't many of the same people be members of the seed group, leading to message fatigue amongst their connections?

Seems like something out of which you may raise the hopes of many a marketing department, but which ultimately proves impracticable.