Oh god, I feel personally responsible.. he kept e-Mailing me asking if I was the one who died.. now to find out that I may have contributed to the untimely demise...
Duolingo is a language learning app with an Owl mascot. It's infamous for sending constant annoying reminders when you skip it's daily lessons. So the app, and owl, have garnered a legion of haters. These posts are celebrating the death of the mascot.
As far as the joke behind the original "our owl is dead" post from Duolingo... I'm not sure.
Considering the timing, It may be a play on the 'Mr. Peanut is dead' Super Bowl ad from a few years ago.