An infocalypse would be a time when our info or our ideas and knowlage would be done away with, by a crash in it's system.
Much like the fall of Babel.
In Snow crash it is presented in a time when the world is one whole, with roads and burbs and various countries within even America, when the world speaks a primary language (english) and the internet is in an amazing form. But all of this can be fouled by someone who wants to take the world over, by placeing his ideas within the system and killing the hackers, or the keepers of the info. He kills the hackers with a nam-shub, which is a virus, which infects the hackers. It also takes over normal people and makes them easy to influence and use because there is a mother language that is inherent in every person in the world, and snow crash uses that to take over their minds. At least thats the idea it presents.
To sum up the point, an infocalypse is a thing that would destroy a world too wound-up on info and being on the "fast-track", if it is indeed something real.