Aside from most people being bad judges of their own sense of humor, it strikes me that many of these jokes land in the subtle-middle-mediocre swamp. If you're going to do one of these pranks, you've got to go to extremes. Way over the top, huge amounts of self-deprecation (witness aspects of Qualcomm's WolfPidgeon bit), etc.
Slideshare's mistake was two zeros. They should have put on an absurd number of zeros, and made the counter go up while the victim watched, by a lot. Maybe break as it passed a bajillion and shatter as the numbers fall off the page. Now it might be that this isn't that funny either, but at least it's pushing hard on people's expectations and a bit absurd. It certainly would have been self-evident as a prank, which is a pretty important component of any April 1 joke that affects you personally.
yeah, on kgs, www.gokgs.com an online place to play Go, they changed everyone's rank to 9 Dan. It was funny because it was absurd and fun to see such a high rank next to your name. If they'd just increased everyone's rank by two, I'd have thought they changed their ranking algorithm or something, and been sad later.
Slideshare's mistake was two zeros. They should have put on an absurd number of zeros, and made the counter go up while the victim watched, by a lot. Maybe break as it passed a bajillion and shatter as the numbers fall off the page. Now it might be that this isn't that funny either, but at least it's pushing hard on people's expectations and a bit absurd. It certainly would have been self-evident as a prank, which is a pretty important component of any April 1 joke that affects you personally.