Ask HN: How do people know they're being talked about on HN?

Repeatedly on HN, this will happen. Someone will mention something, like for instance someone says something about a16z, and the next thing Mark Andreessen is commenting back in the thread.

Or someone says "X company is bullshit!" and 5 minutes later you see in the thread, "CEO of X Company here.. We're not bullshit".

Some of these people are famous and I would think, not hanging out staring at HN (in these particular threads no less) all day.

How are they being alerted that their company etc.. is being discussed and able to respond so quickly?