Socionics is actually older than MB lol. Some (most?) of it comes directly from Jung himself, on whose work MB is based. It's relatively obscure in the West because Socionics took off in the USSR bloc while MB took off in the West.
It's similar in a lot of ways to MB, but (I feel) it goes more in-depth. At least, my studies of MB and Socionics has led me to conclude that. Maybe I look at the wrong sources for MB but I always felt like it was too vague all-around. And of course Socionics isn't incredibly specific either... I dunno, I just used to look at MB, and then I found Socionics and just found Socionics descriptions more concrete. There's also a lot more complicated interplay between types with Socionics compared to MB.
The site I referenced in the OP, wikisocion.org, has quite a lot of information on the subject if you're interested in more. Though I do want to make a warning note - some of Socionics is definitely outdated, and it's not scientific by any stretch. Jung originally presented it based on some kind of empirically-validated model of the brain that, of course, is not correct. Socionics, like MB, should not be considered scientific