The ancient Roman equivalent for posting something and then sharing it via the social networks was called publicare et propagare. Their equivalent of blogging was writing daily letters to a friend, preferably one who lived at the other end of the world, and who had absolutely no interest whatsoever in your letters. Then, once you had a respectable number of posts / letters, you got an editor to copy them, bind them nicely and sell them in a bookstore.
The collections of letters that survived to this day are considered one of the best sources of information about the ancient Rome, though we shouldn't forget that they were written with the intent of being published from the beginning, they're not as innocent and spontaneous as the author wanted them to appear. Just think that, 2,000 years from now, researchers will get their information about our society from archaeological remains, a handful of books and a couple of blogs that survived miraculously.
Something that would have been more useful for us, but did not survive, was the ancient newspaper. In Rome, it was called Acta Diurna, and it evolved from a serious publication into a nice tabloid. Can you imagine a tabloid without pictures of semi-naked wannabe starlets?