It can have a nerdy connotation, but +1 is pretty well known. The average person probably sees it in the party invite/club culture context where you vouch for one person, much like you'll be vouching for sites.
Probably not better than Like, but definitely not worse than Tweet.
Edit: And the ultimate arbitrator of slang agrees: http://www.urbandictionary.com/define.php?term=plus%20one