Back in the mists of time, Companies on Facebook would run promotions where you could share their content in order to enter a competition - effectively getting customers to advertise their product in exchange for the chance of winning one. Advertising costs were slashed, and everyone was happy.
Then Facebook said to themselves, "Hang about, why are we letting customers advertise products for free, when we can totally charge them for that?" So they added the option for companies to pay them money in exchange for increased visibility.
The Companies said to Facebook, "You are having a laugh, why would we pay you to do this when we can get punters to do it for free?" and continued with their viral-style promotions.
The cunning Facebook replied, "Because we have made it illegal under our terms and conditions. Personal Timelines must not be used to administer promotions. See, right there."
"Oh," swore the Companies, and scratched their head for a way to get out of this one.
And then one clever rogue said to his fellows, "Ah, we cannot any longer entreat users to share our photographs to their friends. But look, here is a thing: Over the last few years people have been trained to scan blocks of text for important words. If we put the word 'Share' in quotes, while talking about how to enter our competition, they will think they have to share the photo. Even if we do not say any such thing, and are in fact telling them to comment on it, there will be a good proportion of users out there who just reflexively click 'Share'. All we need is a percent of a percent and we've saved our advertising budget for the year."
And so they did.