Oh, maybe it's not exactly like facebook or Twitter, but it ticks so many other boxes.

I could list many, and argue about them (arguing: another staple of social media), but the most important to me is that HN is effectively a procrastination tool, and it's associated with that mental "fix" of instant gratification.

It's different from facebook in that HN is more heavily moderated and more focused, but other than that, it triggers the same kind of (bad) habits, at least in my case.