At a time when many of Facebook’s earliest and vested employees are leaving, designer Aaron Sittig returned to the company last month to serve as product architect. He left last year for six months and came back last month.

It’s a strong vote of confidence in the company as it begins a new phase of life with a move to a larger, more corporate campus in Menlo Park this summer and amid feverish speculation about a potential initial public offering in 2012.

One of the most widely respected young designers in Silicon Valley, Sittig came to the company nearly six years ago through Facebook’s former president Sean Parker. At first, Mark Zuckerberg was skeptical of what he could bring to the table because Sittig was feeling burned out and unmotivated at the time.

In David Kirkpatrick’s book, “The Facebook Effect,” Zuckerberg kept complaining about Sittig’s work ethic, repeatedly asking, “Who is this guy? He’s worthless. He doesn’t do anything.”

Sittig eventually became one of Zuckerberg’s closest confidants and went on to design the icons for the “like” button, photos and wall posts. He also came up with the idea to tag friends in photos — a killer decision that catapulted Facebook past much larger photo-sharing services like Flickr to become the biggest one in the world.

The success of photos crystallized the idea for the Facebook platform in Zuckerberg’s mind after the team realized that power of people and their social relationships could be the groundwork for all sorts of unanticipated and valuable applications.

As design strategy lead, Sittig articulated the visual principles that have guided the site through countless changes over the years. Those include the ideas that the site should be clean, understated and transparent and that it should be universally accessible.