You can make the greatest looking for group tool in the history of MMO’s but the overwhelming majority of people are not going to use it. The reason is pretty simple, most people do not like pickup groups as they tend to not be as rewarding as grouping with people they know. You have no idea how the quality of the group before you join and most of the time it will be worse than if you group with the same people.
But what if you gave people a reward for joining groups using the looking for group tool. Better yet, a reward for joining groups with people they normally don’t group with using the looking for group tools. For instance let say if you group with people you normally don’t group with dungeons have a 25% better change of dropping loot, and/or higher XP rates. In WoW you could even add a whole category of Social achievements. Grouping with 1000 players, you don’t know, killing so and so with 50 different players, and so on.
By doing this you encourage people to form pick up groups, to group with new players, with out forcing it on. And I think this is very important for the overall health of MMO’s. Most people end up quitting because either A they can’t get groups, B it takes to long to get groups, or C the people they used to group with have quit the game.