In any roleplaying game the living campaign world exists as a shared construct between the players and the gamesmater. There are only two ways to interact with this world - either via the GM or through discussions between the characters. Each game session is an ongoing dance as a player takes centre stage, performs their actions and moves aside to allow another player space to do something.
In a combat based game there is an in-built mechanic to facilitate this. The combat round divides time into few-second intervals and moves forward as each segment is completed, giving everyone a turn. Unfortunately no such abstration exists for a non-combat game which presents a challenge ensuring that all players get their time in the spotlight, especially since it is very possible the characters are in different places, doing entirely unrelated things and circumstances will naturally give a lot more promenance to some characters. This can leave others with little to do which can easily result in unengaged and disinterested players.
In my current game I've been breaking the day into three (morning, afternoon and evening) to give a similar structure to the combat rounds, giving everyone their turn for action. This approach has certainly helped me avoid forgetting anyone and has provided opportunities to switch focus between players during the session but is not enough on its own; I've been aware of an imbalance for some time and have been seeking a way to change it.
A purist would again say that the GM's job is not to be an entertainer, but it is definitely the GM's job to ensure that the game works for all present and if one or more of the players are being left out then the GM is failing in this task. It is also important to see the difference between an overly enthusiastic player monopolising the spotlight (a problem for all, but caused by the player) and the unfolding plot continually pushing one player forwards (which is caused by the GM and is up to him to stop it becoming a problem).
In an attempt to give everyone more and better quality time in the spotlight I've been trying to cut down on the daily minutiae - encounters which really don't go anywhere and just serve to consume time. Non-encounters have their place, building atmosphere and helping the characters feel part of the world, but they need to be saved for bulking out a player's game time when they have had an otherwise quiet session, not played out as routine.
I've also been looking closely at the plot. As well as changing the way I'm handling villains, I've been trying to move circumstances to the point where the players have something approaching a common goal. This will encourage the characters to talk to each other, allowing the players to interact with the world without the GM and can happen while another player is talking to an NPC - effectively creating another spotlight which magically increases the amount of time a player is engaged in the same length of session. The problem I've been having here is that while events may well be linked behind the scenes, that needs to somehow be uncovered before it makes any difference in the game sessions.
Fortunately, an unexpected (and insightful) move from one of the players solved this problem for me, finding linking information and joining some of the dots. This is my favourite kind of development as a GM as it wasn't a reveal handed to the players to move things forwards; it was an in-character idea that was followed through intelligently. By revealing the NPC's background his relevence to the plot was partially discovered which in turn uncovered more of the goings-on in the world. Importantly, it also shifted some of the key story activity to a new player which should help greatly in re-balancing the time in the spotlight and, coupled with less clutter in the sessions and (hopefully) more inter-character exchanges should help everyone be more a part of the game.