Internet. Serious Business.
"Internet. Serious Business." is a highly popular meme. It evolved from "The Internet is serious business." and has since transformed into any word or phrase followed by "Serious business.", or even just "Serious Business" with no preceding phrase.
The meme comes from the belief that what happens on Internet forums (message boards, lists, Usenet, etc.) is usually inconsequential and not worth dwelling on; used as a sarcastic observation on an overheated thread, or as a hint to the original poster that they should drop the subject and move on.
Newfags might try to act cool by using srs bsns or srs bznz.