I have no idea, but it's the charity you get when you give in Givewell's EIN.
The Givewell website also has this as the footer:
GiveWell, aka The Clear Fund (a tax-exempt 501(c)(3) public charity) 182 Howard Street #208, San Francisco, CA 94105
[Bolding mine]
http://www.reddit.com/donate?organization=582565917
"Today we are announcing that we will donate 10% of our advertising revenue receipts in 2014 to non-profits chosen by the reddit community. Whether it’s a large ad campaign or a $5 sponsored headline on reddit, we intend for all ad revenue this year to benefit not only reddit as a platform but also to support the goals and causes of the entire community."