It’s 10 o’clock: Do you know where your kids are online? The answer is probably Tumblr.

Tumblr, the New York City startup that faces a crucial year of incorporating advertising into its scheme, is still under the radar for marketers, but not for the youth, who give the social blogging platform a demographic edge.

In fact, the site turned heads in Silicon Valley last week when a prominent investor commissioned a survey that found Tumblr is even more popular with 13- to-25-year-olds than Facebook.

Garry Tan of Y Combinator, on the hunt for what’s hip among youth, found 59 percent of the young set responded that they were regular users of Tumblr, compared with 54 percent who regularly used Facebook.

There has been no doubt about the site’s popularity, but to top the leading social network in such a key category, even if in just one survey, is noteworthy.

“Marketers are definitely looking to other social networks beyond Facebook,” said a spokesman for research firm eMarketer. “Not because Facebook is ineffective but because the digital audience is more fragmented than ever before.”

Tumblr, which is used for sharing photos on a blog-like platform, cracked the 50 most-visited websites list last year. Still, only 30 percent of marketers considered advertising there, according to Awareness Inc.

Sam Hamadeh, chief executive of PrivCo, said Tumblr has to scale its advertising to meet its audience and its whopping $800 million valuation.

Tumblr CEO David Karp has hired Lee Brown as global head of sales for the company’s 12-person team.

Advertisers still need convincing that the platform is for them, according to Frank O’Brien, CEO of marketing firm Conversation.

“Tumblr is like a bootleg Pinterest,” O’Brien said. “I’ll just go to the real thing versus a wannabe.”