Horwitz announced the move via a tweet with a link to a Facebook post.
After four awesome years at Twitter, I'm moving on: https://t.co/Ya2Q4Ohqv7— Rachael Horwitz (@RachaelRad) May 8, 2015
Her Facebook post reads: "Some quick personal news: After four years at Twitter I'm moving on. I can't say enough good things about the extraordinary people I had the good fortune of working with while there. I'm leaving proud of what we accomplished and will take with me amazing memories and, more importantly, friendships.
"And I'm super excited about where I'm headed next. I'm doing product communications for... this product. I'm joining Facebook this month. Woohoo! I'm really grateful for this opportunity (thanks [Facebook VP of technology communications] Caryn Marooney!). I can't wait to get to work!"
Before joining Twitter at its San Francisco HQ in 2011, Horwitz spent almost a year at Seattle-based consumer engagement agency 206 Inc. She has also been an account manager at Cutline Communications and senior account executive at FleishmanHillard.
PRWeek contacted both Facebook and Twitter for comment but neither responded at the time of publication.