The StoryNet is for general stories that have no direct relation to either division or (usually) the Vast Empire as a whole.
As to why you only see Army people posting on it, I have no idea. I guess only Army people are doing personal stories?
ValthirJourneyman of the Dark Jedi Order
Privateer of the Osk Company
Assistant Squad Leader of Blackjack Squad
ASL/SSG Valthir/3SQD/1PLT/1COM/1BAT/1RGT/Tadath/VEA/VE
TRN/JRN Valthir/Lopen/DJO/VE