There are no real barriers to blogging or using an RSS reader if you want to. (RSS feeds are less ubiquitous than they used to be but they're still common.) And, of course, for saving and sharing bookmarks, it's Pinboard that's buying del.icio.us.

I never really tried out Wave but Knol suffered from the problem you'd expect if everyone gets to write their own self-promotional competing article on a topic.