Mark Zuckerberg dropped a hint about another multibillion-dollar opportunity Facebook hasn't even started to tap yet

During Facebook's Q2 earnings call, investors focused most of their monetization questions on video, Instagram, and Messenger. But in Mark Zuckerberg's opening remarks, he dropped a new stat that also perked some interest.

People now make 1.5 billion search queries on Facebook per day, and the site has indexed more than two trillion posts.

Right now, most of those searches are for people or posts, as CFO David Wehner clarified on the call, but that big number glimmers with potential to analysts.

"While most FB searches are not commercial today, we think the types of searches can evolve over time (recommendations?)," analysts from Bank of America / Merrill Lynch wrote in a post-earnings note, "And we see a $5 bn opportunity assuming $0.01 average incremental revenue per search with search ads, which is a fraction our estimate per search for Google."

With that estimate, BAML sees the monetization potential of search as even greater than the value of Instagram, which it pegs as a $4+ billion opportunity.

When an analyst from Citi asked about the monetization potential of search on the earnings call, Wehner remained fairly coy, reiterating that most of searches are for people or posts, and while there's "potential for there to be commercially relevant content" in those searches, that's not the use-case its seen so far. Instead, Facebook currently sees search as the best way to help people find more relevant content to keep them on the social network long. More time on the site means more time to see News Feed ads.

In Macquarie's post-earnings note, analysts highlighted search as one of Facebook's 6 large opportunities, noting that "it will evolve over time."

Zuckerberg seems to think so too: During Facebook's Q2 earnings back in July 2014, he said that "search for Facebook is going to be a multiyear voyage."