SAN FRANCISCO (KRON) — Twitter CEO Jack Dorsey has resigned, the company confirmed.

He is succeeded by Twitter’s current Chief Technology Officer Parag Agrawal, according to The Associated Press. The change went into effect on Monday, after news of his resignation was first reported by CNBC.

Dorsey was a software engineer before taking over the reigns of the social media platform he co-founded, according to a company blog post from 2008.

In an email announcing his resignation, Dorsey said it was his decision to resign and that he owns it. “I’m really sad… yet really happy,” he said.

Agrawal, who joined Twitter 10 years ago and was most recently its chief technology officer, said he’s seen the ups and downs of the company, but he also sees the company’s “incredible impact, our continued progress, and the exciting opportunities ahead of us.”

Twitter stock jumped by more than 10% at the opening bell Monday, according to The Associated Press.

Dorsey sent the first ever tweet back on March 21, 2006 – it said “just setting up my twttr”.

He also founded and leads Square, a payment app.

The Associated Press contributed to this report.