However, we can make some educated guesses:
* Early Bulletin Boards: The earliest forms of online communication were bulletin boards. These often allowed users to post messages, share files, and engage in discussions. Some of the earliest known bulletin boards were developed in the 1970s, and while we don't have precise records of their content, it's likely that early users shared a variety of things, including personal messages, technical documents, and perhaps even creative writing.
* The "RFCs": The Request for Comments (RFCs) were a cornerstone of the internet's development. These documents, written by researchers and engineers, outlined protocols and standards for how the internet would function. The first RFC, published in 1969, described the architecture of the ARPANET, the precursor to the modern internet. It's reasonable to assume that these RFCs, being essential for the internet's development, would have been among the earliest documents shared publicly.
* University and Research Institutions: Early internet usage was heavily concentrated in academic and research environments. Universities and research institutions were at the forefront of internet development, and it's likely that they shared research papers, technical reports, and other documents on early online platforms.
* Early Websites: As the World Wide Web emerged in the early 1990s, web pages began to be created and published. While we can't pinpoint the very first website, it's probable that these early websites would have contained a variety of content, including personal information, news articles, and even rudimentary forms of e-commerce.
In conclusion, while we can't pinpoint the *very first* document posted on the public domain of the internet, it's likely that early postings consisted of a mix of technical documentation, personal messages, research papers, and perhaps even the first forms of online communication. The internet's early days were a period of experimentation and rapid growth, making it difficult to definitively say what came first.