Randi Zuckerberg, the sister of Facebook co-founder Mark Zuckerberg and the director of marketing for the social network, is leaving to start her own company, Facebook said Wednesday.

"We can confirm Randi has decided to leave Facebook to start her own company," a Facebook spokesman said. "We are all grateful for her important service."

Technology blog All Things Digital said Zuckerberg was leaving to start a "new media firm to help companies become more social" called "RtoZ Media."

In her resignation letter, obtained by All Things Digital, she said she has spent the past six years at Facebook "innovating and pushing the media industry forward by introducing new concepts.

"We have made incredible progress, but there is still much to be done and other ways I can affect change," she continued.

"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."

Zuckerberg said Facebook will "clearly be a central element in all of my projects" and that she would "continue to be a strong, vocal evangelist for the most incredible social platform ever created."

Facebook was founded in 2004 by Mark Zuckerberg while he was a student at Harvard University. Membership of the social network recently topped 750 million.