Edit: Now that I've said it, I don't think the 25% is an accurate percentage of ad funds that go to the mods, as I can't find it in the original thread. However I vaguely remember it from a similar discussion
The amount that each mod gets is public information, they are paid to the address in their profile so anyone can look for themselves how much each mod gets.
Reference: https://bitcointalksearch.org/topic/advertise-on-this-forum-round-180-1560162