Twitter announced on Monday that Jack Dorsey, on of the company’s co-founders and current CEO of mobile payments startup Square, will take over as the new CEO of the social media giant.

It’s a decision that looked unlikely just a few months ago as Twitter said it was looking for someone who could make a “full-time commitment” to the company. But that criteria changed during the search as Dorsey stepped in as interim Twitter CEO, simultaneously leading the two major tech companies.

“I honestly didn’t think we’d land on Jack when we started unless he could step away from Square,” Twitter’s other co-founder and former CEO, Ev Williams, wrote in a blog post on Medium. “But ultimately, we decided it was worth it.”

Williams went on to write that Dorsey is “exactly what Twitter needs at this time.”

Twitter co-founder Ev Williams
Twitter co-founder Ev Williams

“It’s a cliche to say tech companies need to be founder-led, just because most great ones are,” Williams wrote. “We can’t run an A/B test and see what Twitter would have become had Jack stayed in the role from 2008 or I did from 2010. There are so many ways to screw things up, the odds are against us being as successful as [previous CEO Dick Costolo] has been. Nor can we know what things would look like had we stayed in the company with Dick leading (Google-style).”

This certainly won’t be an easy job for Dorsey. Twitter is now a household name, but its user growth has flatlined recently.

Dorsey says his top priorities in taking over will be to execute more effectively, make Twitter simpler to use for new members and better communicate the network’s value to the public.

“Today is a new day. Twitter is bigger and more important to the world than we ever dreamed when we started,” Williams wrote in his post on Medium. “And Jack has already demonstrated the ability to inspire the team and think boldly about the next phase of Twitter. His connection to its roots informs a depth of vision and authenticity of voice, which I’ve been excited to hear.”

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