Web 2.0, a concept created by Tim O'Reilly in 2005, is useful. But it's only a concept, not a technology.
It's now possible to see progress within this concept. I suggest three defined phases, though there's much blurring at the edges.
Web 2.1: personal publishing
Blogs can be dated back to the late 1990s, but the power of personal publishing became evident around 2005 (WordPress and Typepad having both started in 2003).
In Web 2.1, individuals gained the power of the printing press and we saw the emergence of the conversational web through comments and trackbacks. Yet in a world where everyone can be an author, how many readers does each author attract? The answer is very few, except that the 1-9-90 rule puts blog authors as one in a hundred. So each logically has 100 readers, on average. But then power-law distribution kicks in, leaving most blogs with few readers and a few blogs on a par with major newspapers. So the true democratising power of personal publishing has never been realised.
At the time of Web 2.1, blogs were criticised for being hurried and inarticulate. Ironically they are now condemned for being slow and wordy (blog entries look like nineteenth century novels compared to 140 character tweets).
Web 2.2: user generated multimedia content
The next phase introduced audiovisual improvements and the quality of social communications.
Podcasting became popular with content creators, though podcasts have been a niche rather than mainstream habit amongst users. YouTube, from 2005, has been the major publishing phenomenon and the 'broadcast yourself' slogan sums up the spirit of the age.
How are podcasts and videos social? They are very conventional in terms of up-front editing skills, but become social through the filter of recommendation. Few people (beyond bored students) wait on YouTube for something interesting to come along. But everyone gets to know about the major hits through their social networks.
Social networks have always existed (we're tribal animals), but Web 2.2 introduced MySpace, Bebo (remember them?), Facebook and others including professional network LinkedIn and specific Ning-based networks. These are walled gardens with pleasure deriving from connections with and content from our 'friends'. The value of the network increases as members create and discuss content.
Web 2.3: the mobile web
The latest phase is characterised by the rise of the mobile internet. The Flickr photo sharing network belongs here (since digital cameras are universal on phones and click-and-share is a better description of how we use photography than click-and-print).
But the outstanding example of Web 2.3 is the twitter microblogging network. It takes personal publishing and conversations onto the mobile web and beyond the walled garden. Twitter isn't for everyone (it requires obsession, consumes time and poses security and privacy risks) but it has lowered the barriers to entry.
Nor does Twitter work well in isolation: Steve Rubel talks of a hub and spoke strategy in which Twitter can be used as a push channel to encourage visits and conversation at hub sites such as blogs. If Rubel's right, Web 2.3 is about 'lifestreaming' – the converging of all our social media footprints into one constantly updating stream.
If, like me, this gives you a headache, then remember that these phases add to but don't necessarily replace the previous ones. We're living in a rich media landscape with a full spectrum of channels from private, one-to-one conversations through to many-to-many discussions and many-to-one broadcasts. Enjoy it all if you can – but also be prepared to switch off as much as you can.