Jon Stewart's departure after 16 years at "The Daily Show" hits an already struggling Viacom.
Stewart helped turn the company's Comedy Central into a TV force, and now it doesn't have any natural successors for him. Last year, it lost Stephen Colbert, as well as John Oliver.
The departure of Stewart, who didn't announce his plans, fuels investor concerns that carriers will drop the channel and advertiser interest will plummet further.
This past quarter, Viacom's ratings slump dragged domestic advertising revenue down 6 percent. This decline comes as ratings have dropped at MTV and Comedy Central, which saw ratings in the key 18-49-year-old demographic decline more than 10 percent.
Even Stewart's ratings have declined this year, thanks to new competition from Jimmy Fallon's "The Tonight Show." This year, Stewart's viewers in the most valuable demographic—18-34-year-olds—has dropped by 22 percent. It's still larger than Fallon's, though NBC's "Tonight Show" is making gains. (Disclosure: NBCUniversal is the parent of NBC, CNBC and CNBC.com.)