Facebook CEO Mark Zuckerberg said on Tuesday that the performance of his company’s stock since its public offering “has obviously been disappointing.”

Facebook’s stock has lost half its value after one of the most anticipated stock offerings in history. The IPO priced at $38, but shares soon fell sharply.

Zuckerberg spoke at the San Francisco Disrupt conference organised by technology blog TechCrunch.

Speaking in front of an audience, he told TechCrunch founder Michael Arrington that the pressure over the stock offering is “not like the first up and down we’ve ever had.”

The social networking site has nearly a billion users and counting. But the company’s challenge is how to make money from it.

Zuckerberg likes to bring up Facebook’s core mission making the world more “open and connected.” But investors want to know how Facebook will be a lasting business and how the company, created in 2004 with desktop computers in mind, will become a true mobile-first pioneer.

Investors are worried about post-IPO “lock-up” expirations that allow early investors and insiders to sell their shares.

In his letter to shareholders ahead of Facebook’s IPO, Zuckerberg had stressed that “Facebook was not originally created to be a company.”

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