The issue is whether you cause any injuries, loss, or damages to others by the hoax. There's no intrinsic law against lying--the law is against causing damages. That means the potential ramifications are based on 1) what happens; and (2) whether it was reasonable for people taken in by the hoax to have been injured.For example, say that as part of the hoax, you menation that "strange radiation readings" were found at a location. Since  people justifiably get worried about radiation--especially given what's happending in Japan--if anyone panics and crashes while driving away in a panic, if your post was sufficiently believable or well-executed so that the panic was not unreasonable, that could lead to litigation against you.