Facebook co-founder Dustin Moskovitz (left) and colleague Justin Rosenstein (right) said this weekend they are leaving Facebook to start their own company.

The moves are just the latest in a steady exodus from Facebook, a trend that has sparked questions about whether the popular social networking company’s culture is changing and whether it is losing its magnetism.

The new company will build tools to facilitate collaboration within the enterprise, according to the duo. Inspired by the challenges the two had at Facebook as the company grew, the project will use Facebook Connect as its user authentication system. They are both at Facebook for another month, and haven’t actually started the company yet, Moskovitz said in an email. Moskovitz gave us permission to quote his and Rosenstein’s notes below.

But first, just a year and a half ago, Rosenstein was writing how special Facebook was. Given his words at the time, you’ve got to wonder whether anything at Facebook changed that made him move on:

I am writing this note to spread Good News to all the friends I haven’t already overwhelmed with my enthusiasm: Facebook really is That company. Which company? That one. That company that shows up once in a very long while — the Google of yesterday, the Microsoft of long ago. That company where large numbers of stunningly-brilliant people congregate and feed off each other’s genius. That company that’s doing with 60 engineers what teams of 600 can’t pull off. That company that’s on the cusp of Changing The World, that’s still small enough where each employee has a huge impact on the organization, where you think about working now and again, and where you know you’ll kick yourself in three years if you don’t jump on the bandwagon now…

That was then, this is now:

…I’m really happy I took the job. I’m thrilled with the time I’ve had at the company, and with the incredible peers I’ve gotten to know and work with. But something else exciting happened in the year and a half since I joined Facebook. I started spending a lot of time after work talking to Dustin. Efficiency-through-software was dear to his heart as well, and we would stay up til 3am raving about how shortcut keys and high-level abstractions would Change The World. We shared a passion for technology, for entrepreneurship, and for using them to solve the same set of problems.

As our visions for how productivity software could work came into alignment, we thought about building it inside of Facebook. It was an attractive option in many ways, and neither of us was eager to exit a company that was in such an exciting phase of its development. But at some point it became clear that doing so wouldn’t be good for Facebook or for us. Facebook needs to continue its mission of making the world more open through social software, without distraction, and the new project requires a company built around it from the ground up, with the goals of efficiency and group collaboration embedded deeply into its DNA from day 1.So we’ve decided to leave Facebook (in about a month) and start a new company, to build an extensible enterprise productivity suite, along with a high-level open-source software development toolkit, built for the Web from the ground up.

We see this new venture as very complimentary to Facebook. We hope our products will become to your work life what Facebook.com is to your social life. Our software will use Facebook Connect as the default option for identity and authentication. Our user interface will adopt many of Facebook’s conventions, creating a seamless and familiar experience for current Facebook users. And if our new development tools turn out to be useful, we hope the Facebook engineering team will come to adopt them.

Leaving Facebook makes me sad, but I feel I have to follow my passion