Final answer:
The 6th category of social media is online forum sites, like Reddit and Quora, which are platforms for topic-based discussions.
Step-by-step explanation:
The 6th category of social media mentioned by Dollarhide and Drury is online forum sites. These platforms are spaces where users can engage in discussions and post content about specific topics. They often host a wide range of subjects, from tech support to hobby groups. Examples include Reddit, which is made up of communities known as subreddits that tackle endless themes, and Quora, where people ask questions and connect with others who provide insightful answers and perspectives.