Context: Twitter is currently rebranding it self as X.com, which is the bottom logo. X.org is a FOSS implementation of the The X Window System which is being slowly replaced by the newer Wayland display server. Because of the Twitter rebrand Someone decided to make Wayland.social, to parody the X.org to Wayland transition.
And for those wondering, it's a "mathematical double-struck capital x".
I kinda wonder what makes it "mathematical"
In set theory, this style, blackboard bold is used to represent "special sets", for example the set of all integers