Within the long letter Zuckerberg and Chan penned to make the announcement, is also the news that they plan to donate 99% of their ownership in Facebook (FB)—currently worth about $45 billion—to the Chan Zuckerberg Foundation. According to the foundation’s Facebook page, the two founded the organization in 2009, which aims to invest in activities to improve areas such as health, education, scientific research, and energy. The donation, once complete, will be larger than the Bill and Melinda Gates Foundation’s endowment, which as of last December is worth $44.3 billion.

“We know this is a small contribution compared to all the resources and talents of those already working on these issues. But we want to do what we can, working alongside many others,” they write in the announcement.

The couple says it will share more details on this in the coming months as they settle into parenthood. Zuckerberg recent said that he plans to take two months of paternity leave following the birth of his daughter.

This isn’t the first time Chan and Zuckerberg have made large philanthropic donations. In November, they said they would donate $20 million to provide broadband Internet access to all U.S. schools, and in October, announced they plan to open a free private school for up to 700 children from East Palo Alto and eastern Menlo Park, Calif., among other philanthropic activities.