Every post is tied to a username and email address, making it personal information, since each poster can be identified. I'm sure they're also tracking further metrics such as IP addresses, browser fingerprints, etc. It is immaterial if we from the outside are able to identify users, it only matters if it's possible given the data available to the processor. In this case, it is. Not to mention, there is a good chance texts and posts themselves contain plenty of personal information, such as linking to other user profiles, mentioning and discussing people, etc.