Zenimax, owner of id Software, is suing Facebook for $2bn. It alleges that John Carmack, co-founder of id, took intellectual property belonging to Zenimax when he left the firm to join Oculus as its full-time chief technology officer.

Mr Carmack is a legend in the games industry, responsible for pioneering the "first person shooter" genre with titles such as Doom and Quake.

Zenimax also alleged that evidence supporting its claims was destroyed.

Pioneering tech

The arguments are split like this: Facebook says the Oculus team is responsible for creating innovative hardware - its Rift headset - and bringing virtual reality to the masses.

But Zenimax says the Oculus team wouldn't have been able to do any of that were it not for its help in the early stages in developing the software that made the headset capable of doing anything.

Mr Zuckerberg, in testimony which took up most of the day's session, disputed the significance of Zenimax's role in the early days of Oculus.

"Like most people in the court, I’ve never even heard of Zenimax before," he said.

This may play into the hands of the prosecution, however.

Lawyers for Zenimax accuse Facebook of not carrying out due diligence when purchasing the company. The huge deal apparently happened very quickly over a single weekend.