As a history major, I was taught to place high importance on primary sources and firsthand accounts. However, I have recently begun to wonder if trusting personal letters or family stories is the equivalent of someone 50 years in the future blindly accepting the accuracy of Facebook posts, Tweets, etc. Primary sources written by invaders/colonizers are also problematic, for obvious reasons, especially in cases where only their side is well documented. In light of these concerns, how trustworthy are primary sources?
Professional archivist here. It's important to underline the distinction here between important and accurate. Primary sources, as you allude to, inherit the flaws of their creators through their own ideas about the world and how it works. This is why, as an historian, you can get closer to the truth by performing high quality primary source research and balancing these opinions and ideas that emerge with high quality secondary sources that either validate or argue against what you have found. Primary sources are extremely important and vital to historical research, but they are not always accurate, complete, or unbiased. It's your job to add your voice to the conversation by using primary sources to illustrate a new or useful idea or topic of research.
Yes, in 50 years people will probably be looking at our Facebook and Twitter posts to reveal something about this period of history, but they will only reveal information about us as individuals and groups and how our ideas and beliefs shaped the world as we saw it, and not as it actually was.