“It is a very dangerous time,” said Om Malik, an investor at True Ventures whose tech news site, Gigaom, collapsed suddenly in 2015, portending the flurry of contractions.

The trouble, the publishers say, is twofold. The web advertising business, always unpredictable, became more treacherous. And website traffic plateaued at many large sites, in some cases falling — a new and troubling experience after a decade of exuberant growth.

Online publishers have faced numerous financial challenges in recent years, including automated advertising and ad-blocking tools. But now, there is a realization that something more profound has happened: The transition from an Internet of websites to an Internet of mobile apps and social platforms, and Facebook in particular, is no longer coming — it is here.

It is a systemic change that is leaving many publishers unsure of how they will make money.