Verb, intransitive and (less commonly) transitive. Kiss, especially in a vigorous or passionate manner; make out. Thus, Martin Amis, The Rachel Papers (1973): “They were enjoying a kiss—well, more of a snog really.” American popularity was certainly aided by Bridget Jones’ s Diary (1996), Angus, Thongs and Full-Frontal Snogging (2001), and other trans-Atlantically popular examples of British chick lit. Google Ngram.
And when couples do start smooching or snogging, they usually get interrupted — partly because [schlock Hollywood director Andy] Milligan throws so many prying characters into the cage of his film frame. (Richard Corliss, Time, November 30, 2011)/We already know that Matt Damon will be playing Michael Douglas’ gay lover in the upcoming biopic Liberace, but now they’ve both confirmed there will be a good bit of guy-on-guy snogging for the big screen. (Tampabay.com, March 23, 2011)
notoneoffbritishisms.com 
The scary thing is when Americans use it wrong:
http://separatedbyacommonlanguage.blogspot.com/2010/04/snogging.html
Ew.
I’d never heard of snogging until my kids became Harry Potter geeks. Harry and the Potters have at least one song about it. (But they’re American, so it might not count.)
Snog song works nicely anagramatically.
I think it’s an ugly word. It fits right in with words like slime, slither, squirm and sludge.
I rather like snog, it seems a bit less salacious than “make out” and noting that adults are snogging seems to me less junior high than using making out.
What about “shag”? Austin Powers made it famous, and it’s a useful word, since it means the same as “fuck” while not getting you in trouble!
“Shag” has been used in the US South for decades before Austin Powers dragged it into the mainstream. There was even a dance called “the Shag” which was the Lambada of its day…hence the name.
Snog is one hundred percent UK. It would sound beyond ridiculous if a Canadian were to “snog someone’s face off” XD
I actually like the slightly icky, ugly sound of the word snog. I think it fitting that the word itself should sound moist and awkward: a snog is certainly a sort of making out that makes others uncomfortable as a PDA.