On paper, he was just another developer at Facebook. In reality, his influence on Facebook and its future is much larger. Joe Hewitt, who built the initial version of Facebook for the iPhone, left the Palo Alto, Calif.-based social networking company Friday. It was his one-man effort, which then turned into a full-blown revolution, and led to carriers clamoring to tie-up with Facebook. Hewitt announced his exit from Facebook on his blog.

Today was my last day at Facebook. Normally when I leave a job I go out cursing the management and wishing I had left much sooner. In the case of Facebook, I sent heartfelt emails to all of my managers thanking them for the privilege of letting me work there, and I genuinely meant it. Facebook was the longest I ever worked at one company, and the best employer I’ve ever had.

Working at Facebook was like having my own startup, but with a paycheck instead of ramen. Management gave me the freedom to work on my own ideas, and just like with real startups, some of my projects never made it out of the lab, while others shipped and were huge successes. The brilliance of Facebook management is encouraging everyone to take initiative, take risks, and wear as many hats as you can. I wish more tech companies operated like this.

Having known Hewitt for a while, I can safely say that he means every single word. Not one to give lip service, Hewitt is a straight shooter. He joined Facebook after Mark Zuckerberg & Co acquired Parakey, a company Hewitt co-founded with Blake Ross about four years ago. Hewitt’s contributions to the mobile ecosystem, including his work on Facebook for iPhone, (s aapl) was the main reason we included him the inaugural GigaOM Mobile Influencers list. Here is what we wrote about him:

In the world of software, there are few who qualify for rock star status. But Joe Hewitt, a New Jersey-born programmer, is one of them. He started out at Netscape in 2000, working on the browser’s user interface, after which he went on to help longtime cohort Blake Ross reinvent the Netscape browser as Mozilla’s Firefox. In 2006 he left Mozilla to work with Ross on a new startup, Parakey, that in turn was acquired by Facebook in 2007.

Then when Apple launched the iPhone, Hewitt released iUI, a user interface library that consisted of a combination of javascript and CSS, and allowed for the easy development of web applications that matched the device’s native visual elements. Later he wrote an iPhone-specific version of Facebook, which has helped to both take the social network to the top of the mobile heap and jumpstarted the adoption of social networking services on all major mobile phone platforms. What’s even more surprising: Hewitt is the one-man programming machine behind Facebook for the iPhone.

After spending his energies on the iPhone, he moved away from Apple’s platform and turned his attention to the Android platform. Today Facebook is a mobile monster, possibly working on its own mobile platform. Hewitt says he is going to work on new tools for developers and designers. On his blog, he wrote:

And so, I’m independent now, and I’m going to pour myself into understanding the needs of modern developers and designers, and creating software to fill those needs. There are so many opportunities that I can’t even predict what I will end up building, but I am pretty sure I know where I am going to start. I can’t wait.

Here is a video interview I did with Hewitt back in 2009: