Mark Zuckerberg has been an avid fan of both MMA and Brazilian Jiu-Jitsu for a while now.
And it is a well-known fact that he competed in BJJ several times.

However, he wanted to compete anonymously and went to some lengths to conceal his identity – at least initially.
He shared how he tried to do so in a conversation on the Joe Rogan Experience podcast episode:

I guess I rolled into this tournament and I registered under my first and middle names so people didn’t know who I was.

I had like sunglasses and a hat, and I wore a COVID mask.
It wasn’t until they called our names to step onto the mat that I was like: “All right.”

Took all the stuff off, and the guy [was] like: “Uh, what?”
I think he was trying to figure out what was going on.

Zuckerberg recounted the moment his opponent began to realize who he was:

Afterwards, his coach was like: “I think that was Mark Zuckerberg who just submitted me”, and the coach is like: “No. No way.”

[The other guy] was like: “No, I think that was.”
The coach was like: “What? You’re fighting Mark Zuckerberg? Get back in there, go fight him.”

And he was like: “No, he just submitted me.”

Zuckerberg tries to train as regularly as he can, even though he did experience an ACL injury in training:

It’s really important for me for balance.

I basically try to train every morning. I’m either doing general fitness, or a kind of MMA, and do sometimes grappling, sometimes striking, or some both.

But it got to the point where I tore my ACL training.
At that point, I wasn’t integrated between my weight training and my fighting training, so I think I was probably overdoing it.

So now, I’m trying to do this in a cohesive way, which I think will be more sustainable.