That's a good point. I think a lot of forums were less vulnerable for a number of reasons. They typically don't have a large audience (not all, but most), which makes them less of a target. They're also organized around niche interests that don't intersect much with politics and cultural issues, off-topic forums aside. And they're probably more heavily moderated than social media and blog comments.

I think the general point stands when considering large-scale platforms.