Well the underlying question isn't what the factions are for, that most of us already know. The question is the motive behind each faction, what they represent to the Guardians who serve under them. They are vying for control over certain areas, why? Each faction may have different reasons for controlling a certain area, one may want to use it as a beachhead for expansion of small colonies, other for militaristic, study, resources for those unable to defend themselves, etc. This is something I have wanted to know since I first heard about them. Bungie doesn't even need to go into real depth about it now. As to why it's trending, I assume is that since Factions represent a large chunk of the story behind the competitive multiplayer, and to a lesser extent the campaign, people may want to know who to fight for, who best suits their Guardian to work with.