Typically, the three magic forms rate like this in popularity:
1) pact magic
2) shadow magic
3) truename magic
I'm gonna go out on a limb here and say that the reason for this is because pact magic seems like it is the most powerful and truename magic seems like it is the least powerful. I believe the reason for this perception is because pact magic gives your character a lot of static bonuses that your character can "walk around with," much like the cleric can walk around with hour/level and 10 min/level effects like freedom of movement, death ward, magic vestment, greater magic weapon, etc. The more effects you can pile onto your character the higher the perceived effectiveness.
Shadow magic on the other hand is utility magic. The shadowcaster doesn't "cast the spells that makes the peoples fall down." He casts spells that immobilize or hinder enemies and overcome trials. In that respect he is like an enchanter or illusionist specialist wizard who has prohibited evocation and necromancy. Not a lot of death happening, but a remarkably useful character. He has a lot of tricks up his sleeve that make otherwise challenging situations a piece of cake. For that reason, he is well-liked.
The truenamer is the red-headed step-child of the bunch because of that ever so touchy subject of the DCs that scale with CR. This means that a truenamer's spells are always going to be challenging to cast unless he is casting them on low-CR opponents. The truenamer is every bit as powerful as the others, however, his magic is not as constant/long-lasting as the binder's and not as reliable as the shadowcaster's.