Final answer:
Posts to a user forum on a single topic and arranged in date order are known as a blog, which was part of the user participation focus of Web 2.0 introduced in 1999. Blogs display entries in reverse chronological order and differ from other social media posts in their structure and content delivery.
Step-by-step explanation:
Posts to a user forum with commentary on a single topic, arranged in date order, are called a blog. When Web 2.0 was announced in 1999, it emphasized user participation. This new era included various forms of social media where users could actively engage by posting content, commenting, and sharing. In contrast, a single topic series of posts in a user forum is characteristic of a blog, where entries are typically displayed in reverse chronological order allowing readers to follow the author's thoughts, commentary, or news over time. Social media, on the other hand, involves a broader scope including different types of social media posts on platforms where individuals not only post content but also interact with each other's posts through comments, likes, and shares.