Yep. Until the very recent boom of social networks, everything published is, by definition, the product of the educated and the upper classes. The farther back you're going, the more estranged from ordinary people you are. In the Middle Ages, you'll have nothing but texts about the adventures of saints and kings.

And of course, even with social media, there is still a large bias issue - not just with who is sharing, but also what; most people don't share everything about their lives on social media.