For example:

The growth of [Facebook’s] user base and its ever-swelling page views mean an almost infinite inventory to sell. But the expanding supply, together with equivocal demand, results in ever-lowering prices. The math is sickeningly inevitable. Absent that earthshaking idea, Facebook will look forward to slowing or declining growth in a tapped-out market, and ever-falling ad rates, both on the Web and (especially) in mobile applications. Facebook isn’t Google; it’s Yahoo or AOL.