Facebook HR exec reveals the most essential question to ask in a job interview

The most important question to ask candidates during a job interview isn't the most complicated one.

Tudor Havriliuc, Vice President of Compensation, Benefits and Global Mobility at Facebook, says the query from which he learns the most about a job seeker is simply, "Why?"

"We ask [candidates] about why they want to join," he says. "We also make sure that we really understand what their values are. Do they share the same values of connecting the world, of building social value?"

Havriliuc spoke with CNBC in advance of Glassdoor's Best Places to Work Roadshow, at which Facebook will explore what has made it such a coveted employer.

When evaluating a job seeker, Havriliuc says Facebook's priority is ensuring someone will be energized by the company's mission as well as its unique environment.

It's essential, he says, to probe what drives a candidate to join a company.

"The way that we look for commitment to the mission and the desire to be here, is we really look at people's motivations," he says. "Would they thrive in an environment where people are pretty open and direct, where there is a lot of collaboration? Are they the kinds of people that can articulate what their best day at work has been?"

That last part — the ability to pinpoint a time the individual felt truly engaged and successful at work — is key to finding candidates who will dig into projects and become passionate.

As Havriliuc puts it, "We want people to be doing the things that really excite them."

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