Snog is one of the words I changed from common to rare a couple of years ago, on the grounds that they were common only in some parts of the English-speaking world.
Probably it is becoming more widely known in the US because of Harry Potter and other forms of popular culture. The indexes at the Time and Newsweek sites turn up a few examples, some quoting British people, and some by reviewers of British books or films. Mostly, these magazines don't consider it necessary to provide a definition, which is a sign the word is thought to be fairly widely known. Still, the frequency of use in US publications seems fairly low, so I don't think it's time to make it common again.
