In a letter to Facebook executives Sheryl Sandberg and Elliot Schrage exclusive to AllThingsD, Zuckerberg said that her "goal is to launch my own innovative programming and work with media companies to develop their programming in new, and more social ways." That sounds like Zuckerberg will focus primarily on TV and cable networks though the letter did not specify exactly what the focus the new company will be.
Zuckerberg did say that she will continue to be a "strong, vocal evangelist for the most incredible social platform ever created" and will be happy to advise Facebook on media projects and Facebook Live.
Zuckerberg has been on maternity leave for the last several months. She has not tweeted much in the time with most of her updates about being a new mother. She has not tweeted about leaving Facebook (yet) with her last tweet coming on July 31 about having dinner with Sean Penn and others discussing (presumably) charitable works in Haiti.
Update: Zuckerberg just tweeted about leaving Facebook at 2:39 p.m. PST.
RtoZMedia.com appears to be a placeholder page for Zuckerberg, if indeed that will be the destination of her new company.