The Soviet Union Joins Usenet:

One of the precursors of modern Internet forums was a network known as Usenet, which was created in 1980, and initially linked computer users from North America and Western Europe. So, when the Usenet world received a post from then-Soviet Prime Minister Konstantin Chernenko on April 1, 1984, announcing the creation of the Usenet site “Kremvax,” and an expressed desire to get the U.S.S.R. on Usenet, users were quite shocked. After all, the Soviet Union was a closed society, and wouldn’t including the Soviet Union on Usenet be a national security concern for the U.S.? After two weeks of discussion, Piet Beertema finally came forward to acknowledge that the Kremvax site was, in fact, a hoax. Six years later, when the Soviet Union did actually join Usenet, one of the first sites was called Kremvax, in honor of the hoax.
UPDATE: This is considered by some to be the first Internet hoax.