The fine mind behind the "What-If" column at xkcd tackled the question of how many possible tweets there could be in the English language.
The question is valid because tweets are limited to 140 characters. The number of potentially different English-language sentences, of course, is infinite, because sentences have no set length. But since there's an upper limit on tweets, then the number of potential tweets that make sense has an upper limit. It's a really big upper limit, though, requiring more time than the universe has existed or will exist to read them all.
I will quibble with the author's final conclusion, however. Because of the immense number of potential tweets and the fact that it would be impossible to read them all, he suggests that we will never run out of things to say. As of this time last year, the famous-for-no-earthly-reason Kim Kardashian had 14 million Twitter followers. That means Twitter has already run out of things to say, or at least out of things to say that are worth the time.