Iām not sure. It might depend on what counts as a like. Slashdot had upvoting before Facebook existed. The Wikipedia article for āLike buttonā lists two pre-Facebook implementations, and mentions one was based on Diggās digg button.
Discussion
I saw that Wikipedia bit about the Digg button. Makes sense that the like button (or under another name) existed pre-facebook. There were already discussion forums before that time