Roblox is indeed a MUD, but I would argue that Roblox contains too much varied niche "game content" to meet the needs of kids looking for a platform for mediated socialization. Not enough of the people who are playing Roblox are playing it just to socialize. You get more of the MMO experience (or, I guess, the "social multi-game lobby" experience, ala NeoPets), where no strangers want to stop and talk OOC; they only want to engage with the game elements themselves or with your character via RP. Everyone's there to see or do some novel-to-them thing. Nobody's treating it like an old, familiar place to hang out.

Minecraft doesn't have that same constant flow of novel game-elements to match the number of hours people invest into it; and so most play in Minecraft is a lower-key "post-game" kind of engagement with evergreen mechanics — the same "maintenance" kind of play that you see in e.g. Animal Crossing, together with the "practice" kind of play you see in the speedrun community. That kind of gameplay is extremely amenable to being interrupted for socialization. It's like gardening in a community garden.

(Analogy for socially-mediated entertainment for the pre-MMO-era kids: it's like getting together with your friends with a bunch of individual LEGO sets you got for your birthday to construct them all; vs. getting together with your friends to play with the same big old tub of random assorted LEGO bricks you've had for years. Building the sets favors engagement with the novelty of the work, over social engagement with your friends. Building from a tub almost forces social engagement with your friends, as that's the mechanism through which any consensus arises as to what you're going to be spending your time doing.)

What is a MUD?

Short for Multi User Dungeon, it's a world you can explore and interact with alongside other users. They were generally text-based (like Zork), though I believe there were some roguelikes and graphical MUDs too.

Many multiplayer online games played today have a heritage that comes from the MUDs. Minecraft is certainly one of them and fills a similar niche, though to categorize it as a MUD seems strange, unless any multiplayer virtual world with an ROG-like element is also a MUD.

The link in the grandparent comment is (essentially) a really good and extensive definition of the term. And then also a history of MUDs and what they've since evolved into.