SAN FRANCISCO — Snapchat plans to debut ads after a new fundraising round valued the vanishing photo app at an eye-popping $10 billion.

“People will see the first ads on Snapchat soon,” co-founder and CEO Evan Spiegel revealed Wednesday at Vanity Fair’s New Establishment Summit in San Francisco.

The sky-high valuation puts pressure on Spiegel and co-founder Bobby Murphy — they famously turned down a $3 billion buyout offer from Facebook — to prove that their “sext”-enabling app can make money.

The no-frills ads will show up in the “Stories” feature where users post videos and photos of their day. They will live for 24 hours before going the way of all snaps and disappearing.

“They are not fancy,” Spiegel said. “They are not targeted.”

When Yahoo! News anchor Katie Couric asked if Snapchat had reached 100 million users, Spiegel ducked and said the company has a policy of not celebrating metric milestones.

Spiegel shared a panel with former New York Mayor Mike Bloomberg, who recently returned to run Bloomberg LP, his financial news and information giant.

Bloomberg, who is said to have the Snapchat app on his mobile phone, demurred when Couric asked if he ever sexted with it.

“I can’t answer that,” said the billionaire philanthropist, who was recently made an honorary knight by Britain’s Queen Elizabeth II.

Bloomberg, who was also anointed the “nanny mayor” during his three-term reign, sounded a lot like a cranky grandpa during the session with Spiegel.

The recent boom in tech startups is “not first time that a company with no revs had a very high stock valuation,” said the 72-year-old Bloomberg.

Harkening back to his Wall Street days at Salomon Brothers in the ’70s, Bloomberg said it was a problem then as now: a company with a product in development had a sky-high valuation only to plunge once it began trading.

While Spiegel might escape that fate, he has to be careful how he develops his business, Bloomberg said.

“He seems to have staying power, and seems to not be a flash in the pain,” he said. “He has to find a way to keep his technology better and be au courant.”

Bloomberg, who is known to demand absolute loyalty from his underlings, also shared his advice on the hiring front.

“You should always try to hire people that are smarter than yourself,” he said. “Make sure they have good ideas and the freedom to express those ideas.

“When they don’t work out, they are not on my s- -t list,” he added. “People want recognition and respect, but they may also want to know that you have your back.”