Instagram is hanging on to a demographic that's rapidly abandoning the Facebook platform, according to a new RBC Capital Markets survey, making Mark Zuckerberg's buy of the photo-sharing app "one of the best acquisitions in internet history" according to one analyst.

"We clearly see a fade amongst the 13- to 18-year-olds in usage of Facebook, and I think most people would probably intuitively feel that that's happening," Mark Mahaney, lead internet analyst at RBC, told CNBC's "Power Lunch" Monday.

73 percent of survey respondents between the ages of 13 and 18 had an Instagram account. Only 57 percent had a Facebook account.

Both social media platforms dropped in usage among that youngest cohort in the six months preceding the report, but Facebook took a much larger hit than Instagram. In June, RBC found 81 percent of 13- to 18-year-olds had Instagram accounts and 75 percent had Facebook accounts.