It's a Proper Noun: The most common mistake is to think of “贴吧” as a generic word for any online forum. While it *can* be used more generally in some contexts, 99% of the time it refers specifically to
Baidu Tieba. It's a brand name, like Reddit, Facebook, or Twitter. You wouldn't say “I read a Facebook” when you mean you read a social media post. Similarly, you use “Tieba” to refer to the specific platform. The generic word for “forum” is
论坛 (lùntán).