Just a quick thought before I forget it.
So - a lot of services allow you to grab all of your RSS feeds from all over the shop, and republish them in a central aggregated feed.
The resultant feed - of bookmarks, tweets, flickr pics, LastFM music, blogposts, yada yada - is noisy. REALLY noisy. In fact, unless you know someone really well, it's just too much information and you drown in it.
But there are some people for whom that much information is good, and comforting. I'd keep an eye on everything my other half was up to, for instance - not for stalking reasons, or because I want to surveil him, but because it's nice to know whats going through his head - it's his presence when he's not around - as Leisa would say, it's Ambient Intimacy.
But other people - no, I really don't want to know their every move - I'd like perhaps a once a month update of key items.
So a social aggregator with degree-of-intimacy - where you can pick and choose elements of a person's behaviour to subscribe to. This should couple with a few smart bits at the back which would desubscribe or deemphasise sections of a person's feed according to your consumption behaviour. Not reading all of Friend X's long screeds, but most of their tweets? Eventually the long screeds will drop off your updates.
Facebook maybe goes part way towards this, but it really doesn't understand the shades of grey and changeability of social ties.
Fluidity, and smartness, and the understanding that friendship ebbs and flows.