Etymonline tells us:
best (v.) Look up best at Dictionary.com
"to get the better of," 1863, from best (adj.). Related: Bested; besting.
I'm afraid the authors of etymonline are describing the usage they find, and (luckily!!!) not trying to establish how people should use words, so I cannot (and out of principle, will not) appeal to your hunger for a prescriptive answer.
But as it seems, best has been used as a verb since 1863, which means it predates, as RegDwight mentioned, the internet by a hundred years. It may well be that you have only come into contact with the verb due to the internet, but I doubt the internet itself contributed to the popularity of the verb.
As to the grammatical validity, notwithstanding your love for prescriptive grammar, it does not hold water. You can find a thousand grammarians saying that verbing an adjective is wrong, they will be bested by a population that uses the language and tells them where they can put their "rules". Generations have been taught that "I am good" is not grammatical and others that the verb have is always followed by got. Again others have been told not to use the passive voice, by someone who didn't even understand what the passive voice was. A lot of wasted effort, but if you like that stuff, I'm afraid you may not find as many supporters here as you may have wished.