As Jesse said, what lexicographers do is search out words that people use, see how they use them, and write them up. Adding a new word to the dictionary doesn't amount to giving it a stamp of approval; it just means that the lexicographers found the word in wide enough use over a long enough time that they decided dictionary users might want to know about it.
Well, owing to the Internet, anyone today can figure out how widely used a given word is. Just google it. "Conversate" is all over the Web. If, however, you want to find out whether it's in standard use -- which is often what people mean when they wonder if something is a "word" -- archives of edited media, such as Google News, are a better place to look. According to a search I did just now, "conversate" has turned up in the newspapers and press releases, etc., that Google News tracks exactly five times in the past month. That's very few. Two of the five come from Ta-Nehisi himself; two are from AllHipHop.com, and one was published in an actual newspaper, in a quote from a basketball player. Isn't this already starting to be a good basis for drawing your own conclusions about "conversate"?
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