When you're in the chat and a show has concluded, users may post this strange message 888888 to applaud the performance…

Its origins come from the program DIGITAL HIGH LIFE (DHL), hosted by Mitsuco Delight.

8 in Japanese is 'hachi' （はち）, Japanese onomatopoeia for “applause” is "pachipachi" “パチパチ” , so this could be written as 88 many times in internet slang.
Here is a link that probably explains it better.

Users may also post +++++++++++ to convey the same sense of appreciation