Photo Credit: Reuters
Signal is a well-known privacy-focussed messaging app, that is widely used by security experts, privacy researchers, academics, and journalists around the world. The Signal protocol also underpins WhatsApp's end-to-end encryption, although one key difference is that Signal is open source, while WhatsApp is not.
Use Signal— Elon Musk (@elonmusk) January 7, 2021
As mentioned above, this wasn't the only dig that Musk took against Facebook. A little earlier, he also tweeted a meme connecting the starting of Facebook to a domino effect leading to the violence and chaos in Washington.
This is called the domino effect pic.twitter.com/qpbEW54RvM— Elon Musk (@elonmusk) January 7, 2021
Musk has a history of disagreeing with Zuckerberg on Twitter, that goes back many years now. The two have, for example, famously disagreed on artificial intelligence, that Musk argued had to be carefully regulated. When Zuckerberg had spoken against fear mongering over AI, Musk tweeted that the Facebook CEO's "understanding of the subject is limited."