The internet's biggest businesses have a looming growth problem

• Internet-based businesses have grown strongly for most of the past two decades.
• The industry may be entering a new lower-growth phase now. 
• Digital advertising may be the most saturated, while e-commerce could see growth rates moderate.

After two decades of mostly unfettered expansion, the internet's biggest businesses are entering a new period of slower growth.

That's according to Bernstein Research, one of Wall Street's top tech analysis firms.