Musk took to the platform to troll Meta after the platforms went down, tweeting: "If you’re reading this post, it’s because our servers are working."
He was joined by the official account for X as well, which tweeted: "We know why you’re all here rn."
Many users were worried that their personal account had been hacked.
One wrote: "Not me changing my passwords on FB three times in five minutes thinking I got hacked only to find out that Facebook/IG are down for everyone."
Messaging service WhatsApp, also owned by Meta, did not appear to experience any disruption to its services during the outage.
The disruption began at around 3.00pm GMT, with services back up and running after around an hour and a half of not being accessible.
Website Down Detector showed that Facebook had more than half a million reports of service disruption.
Many users on X took to the platform to share jokes of how many people were arriving to check if they were the only one who'd been cut off.
One person estimated roughly how much money Facebook may have lost during the outage.
Taking Facebook's ad revenue for its websites last year, which was $84.2 billion, that comes down to around $160,000 a minute, or approximately $2,670 a second.