Openness - anyone can join, and anyone can follow anyone else (unless they restrict access to friends who request access).

Immediacy â€“ it is a rolling present. You won’t get the sense of Twitter if you just check in once a week. You need to hang out for minutes and hours, every day, to get in the groove.

Variety - political or technical argument, gossip, scientific info, news flashes, poetry, social arrangements, classrooms, repartee, scholarly references, bantering with friends. And I'm in control of deciding how much of each flavor I want in my flow. I don't have to listen to noise, but filtering it out requires attention. You are responsible for whoever else's babble you are going to direct into your awareness.

Reciprocity - people give and ask freely for information they need (this doesn't necessarily scale or last forever, but right now it’s possible to tune your list - and to contribute to it — to include a high degree of reciprocation; more on this in a moment).

A channel to multiple publics - I'm a communicator and have a following that I want to grow and feed. I can get the word out about a new book or vlog post in seconds - and each of the people who follow me might also feed my memes to their own networks. I used to just paint. Now I document my painting at each stage of the process, upload pix to flickr or flicks to blip.tv, then drop a tinyurl into Twitter. Who needs a gallery or a distributor? You don't have to be a professional writer to think about publics. Anyone who publishes a blog knows that they are not simply broadcasting to a passive audience – blog readers can comment, can link back, can criticize and analyze, and in many instances, can join the blogger in some form of collective action in the physical world.

A way to meet new people - it happens every day. Connecting with people who share interests has been the most powerful social driver of the Internet since day one. I follow people I don’t know otherwise but who share enthusiasm for educational technology, DIY video, online activism. creativity, social media, journalism, Burning Man and public art, teaching and learning, compost, Catalunya, the public sphere, mass collaboration, Amsterdam - the list is as long as my list of interests. Developing the ability to know how much attention and trust to devote to someone met online is a vitally important corollary skill. Personal learning networks are not a numbers game. They are a quality game.

A window on what is happening in multiple worlds, some of which I am familiar with, and others that are new to me.

Community-forming - Twitter is not a community, but it's an ecology in which communities can emerge. That's where the banal chit-chat comes in: idle talk about news, weather, and sports is a kind of social glue that can adhere the networks of trust and norms of reciprocity from which community and social capital can grow.

A platform for mass collaboration: I forgive the cute name of Twestival because this online charity event has raised over a quarter of a million dollars via Twitter, funding 55 clean water projects for 17,000 people in Ethiopia, Uganda, and India. If I wanted to tweet a request, I could offer another dozen examples.

Searchability - the ability to follow searches for phrases like "swine flu" or "Howard Rheingold" in real time provides a kind of ambient information radar on topics that interest me. Twitter users developed the convention of adding a tag with a hash sign in front of it – like #hashtag – that enable them to label specific topics and events. When I recently participated in a live discussion onstage, we projected in real time the tweets that included a hashtag for the event, an act that blended the people in the audience together with the people on the panel in a much more interactive way than standard Q&A sessions at the end of the panel. After years as a public speaker and panelist, I found it fascinating and useful to have a window on what my previously silent audience was thinking while I was talking. You have to be sure enough about what you are saying onstage to keep from being distracted or thrown by the realtime feedback. Backchannel twitterers have been to virtually mob speakers they felt were wasting their attention.