The 31-year-old announced his retirement from the microblogging platform in an emailed memo to staff Monday.

At its peak, Tumblr was an influential social media site where close-knit fandom communities could share writing and multimedia created and curated by bloggers and artists. The network’s presence began to decline following its $1 billion purchase by Yahoo in 2013. Now, Tumblr is under the control of Verizon, which acquired Yahoo in June and merged it with AOL to create a new subsidiary named Oath, which manages Tumblr.

In the memo, Karp said his departure “comes after months of reflection on my personal ambitions,” but did not detail any future endeavors.

Karp’s full memo to the Tumblr staff is below:

Friends and colleagues –

After nearly 11 years, I’ve decided that 2017 will be my last year at Tumblr.

It is truly overwhelming to write those words. But I have the comfort of knowing I leave Tumblr in better hands than my own. Jeff [D’Onofrio] is the most capable and caring leader I’ve ever met, which I say with no exaggeration. Simon [Khalaf] has worked tirelessly to support us and everything that makes Tumblr special. But even more, they are both true believers in what we’ve built and how much we can still do.

I look back with so much pride. At a generation of artists, writers, creators, curators, and crusaders that have redefined our culture, and who we have helped to empower. There are no words, though, that can express how sincerely grateful I am for the privilege of working with you. This team and place has been my family and home for most of my adult life. That I have gotten to spend this time working with people so spectacularly talented and unstoppably optimistic is a blessing I hope you have shared, and will continue to share.

I beg you to understand that my decision comes after months of reflection on my personal ambitions, and at no cost to my hopefulness for Tumblr’s future or the impact I know it can have. The internet is at a crossroads of which this team can play a fundamental role in shaping. You are in the driver seat, and I am so excited to see where you go!

Thank you for allowing me this honor,

David