Earlier in the game's history you didn't have players as divided by powerlevelling penalties as they are now. At the 16 cap you could have four groups covering all level ranges (1-4, 5-8, 9-12 and 13-16). Now it would take six groups to cover all levels at a minimum and in practice you often see ten groups of which none hit your level.
The other factor is that dungeon scaling seriously penalizes people for opening LFMs to PUGgers. Some people still PUG because they are willing to prolong grind for some social interaction, but efficiency minded players do not post LFMs now. It's hard to say how many people play that do not interact with the LFM system.