Randi Zuckerberg, the sister of Facebook co-founder and CEO Mark Zuckerberg, has revealed that she is leaving the company to start her own social media firm.

Zuckerberg, who is Facebook’s director of marketing and has been with the social network for 6 years, is forming her own company called RtoZ Media that aims to help companies become more social.

Here is an excerpt from her resignation letter to COO Sheryl Sandberg and VP of Global Communications, Marketing and Public Policy Elliot Schrage - which you can read in full at All Things D:

I have spent my years at Facebook pouring my heart and soul into innovating and pushing the media industry forward by introducing new concepts around live, social, participatory viewing that the media industry has since adopted. We have made incredible progress, but there is still much to be done and other ways I can affect change. Now is the perfect time for me to move outside of Facebook to build a company focused on the exciting trends underway in the media industry.

She adds that, “Facebook will certainly be a central element in all of my projects. I plan to demonstrate that ANYONE can do groundbreaking media work on the platform.”

Zuckerberg has been on maternity leave for the last 3 months after having her first child. Apparently, being away from Facebook allowed her to have some time to consider the next step in her career.

It remains to be seen who will step up and fill the void left by Zuckerberg’s departure. Whoever it is will have big shoes to fill, as she played an important role in Facebook’s journey to becoming the world’s biggest social network.