True, that's a solid point, thanks for the reply.

However, ad agencies typically get paid more for spending more, so I don't think a "no cost" campaign works well for covering their costs.

Of course there's probably agencies out there that do/will pitch this type of campaign, and charge for the ideation + production. But virality is usually an empty promise.

Alternatively, increasing the k-factor on an already successful campaign is something agencies will want, so I believe they will fork out for these sponsored placements regardless.

All told, I really do think some secret sauce for targeting (aided by Twitter's/FB's data insights) plus some cool content produced by the brands/agencies will win out as the monetization strategy for social networks.