As present day historians have evidence of ancient times due to the recovery of hieroglyphs and symbols on walls and ancient books, future historians would use sites such as twitter and wikileaks for reference.
Though future historians would have access to a lot information through twitter, majority of the information would be irrelevant and since there is no way to sieve out the useful information from the common tweets of the average teenager, it would only result in the massive consumption of time. Also since people know their tweets would be archived i am guessing their tweets would lack some level of integrity. As for privacy concerns? I definitely do not want my descendants reading some of my tweets!