"555" is an alternate sort of version of the Internet
No, I'm serious.
In Thailand, the number "5" is "ha." It is pronounced ha. Thus, "555" is "hahaha," and similar to "laughing out loud."
It originated in the Korean (!) game Ragnarok Online, on the English server. The English server was overrun with Thais, who would use this phrase near-constantly, much to the confusion of the English-speakers on the server. Eventually, the majority of people learned what it meant, and, even after Thais were banned from the server, usage of the phrase persisted in some areas.
It hasn't spread very far at all beyond the Ragnarok Online community, as far as I can tell.