The following part, in particular, got my attention:

Another example of this is what's happening on platforms like Twitter, YouTube and Facebook every day. Brands can engage on each of these in a meaningful way at low -- or no -- cost. ....

It's easy to forget now that these were once considerations that search engines had to navigate through in the days before paid search advertising emerged. At the beginning of the search engine era, marketing was accomplished organically and engines struggled to find ways to monetize the traffic they were sending sites -- sometimes going so far as to consider removing advertisers who declined to buy display advertising from their indexes so that they no longer ranked well for certain search terms.

Part of what search engines offered to attract advertisers to their paid search platforms was access to data, and the ability to target in a variety of ways that would have been more difficult to do organically outside their walls. Most importantly, search engines developed a way of making sure that the advertising opportunities they offered marketers aligned with the way consumers were already using their sites, rather than trying to change behavior instead.

In a similar way, if social media sites want to find a way to monetize engagement, there needs to be some added value for the marketer. What that might mean for platforms is giving up some control over data (something that Facebook has been wrestling with) or the type of interactions that marketers are able to take part in.