IIRC he announced this late 2017 during the bull run and was stated that he's still studying crypto and blockchain. And on latter of 2018, it's on the news that they have been hiring blockchain professionals which is an obvious act to understand that they will join and adopt the tech soon.
Creating a 'facebook coin' is likely but it's still a rumor, they have a large number of users so it will applicable to them. AFAIK, they have created sort of coins before during those restaurant city and farmville days.
I also heard and read about those rumors that Facebook will create its own Crypto Currency, I think it is way back in late 2017's when I heard about it. This may be still a rumor but if the CEO of Facebook itself is studying about crypto and blockchain there might be a higher chance that they will create their own Crypto Currency or their own blockchain or they will use the blockchain technology to improve their platform.