I see 5 major dimensions that can characterise information sharing: individuals, topics, opinions, things and time.
Individuals – this is about people addressing an audience, talking to each other or reading what others write.
Topics – we are interested in specific themes and not others: I am into VoIP and peer-to-peer and not into gardening and pets.
Opinions – what it is all about: information, ideas, thoughts expressed, clashing and leading to others.
Things – for lack of a better word. Food for thoughts, almost literally: news, articles, events, books, new web sites, new products, etc. Anything one can have an opinion about.
Time – the organising principle that makes conversation and evolution possible.