Friends is a place for you to follow the people and things you care about most. Noise is a place for everything else. Friends is for considering things closely. Noise is for more of a scan. But, really they're both just part of a simple duality, offering two ways of looking at things. Use them however you'd like. The people you follow don't know which category they're in, just that you're following them.
Shift + 5 will allow you to toggle the linear Friends feed to Noise's fluid grid, and vice versa.
(@anna at Ello!)