On Monday I went to see author and thinker Clay Shirky talk at a lunchtime seminar hosted by the Demos think tank.
I travelled in to London earlier than I needed to on a crowded train, sitting on a slow bus across town and then squeezing into a bright but too warm room to sit on a hard seat in order to listen to something which was being recorded and will later be available as a podcast.
Clay was charming and intelligent and funny, and I got to hear him thinking out loud about the impact of social tools on international politics, which was fun, but I could have done all that by listening in online, or even by watching the stream of brief reports appearing on Twitter, the communications service that is currently taking the net by storm.
Instead I sat there offering my own online commentary on what he was saying while looking up references on the web as he talked.
What drove me there was the same inner need that got me to the OpenTech conference last weekend, despite the fact it meant a trip to London on a Saturday.
Two weeks ago it took me to York for Shift Happens, an event for those working in the arts to explore new technology, and before that it had dragged me to 2gether08, the strangely-named but fabulously productive convention that arch-networker Steve Moore created by inviting a lot of interesting people to come up with some cool things to talk about.
I am sure that it will take me around the country and indeed the world in years to come, because there is something special about being in the same space as someone else.
However good the video link, however clear the audio, and however anatomically accurate the avatar, sharing the same space and breathing the same air makes a difference to the quality of interaction, especially when you're trying to do something creative rather than being a passive member of an audience.
This doesn't mean we shouldn't try to improve the various online alternatives to being there, just that we should not expect them to be an adequate substitute in all circumstances.
Online may not be the same as being there, but it does of course have its own special advantages.
Online I can participate in distributed events, meet with people who could never all be present in the same space at the same time, and use the tools to mix synchronous and asynchronous communications to allow a distributed form of participation.
I can even be in two places at the same time. A video feed of every session at 2gether08 was streamed live on the internet, so I managed to sit in a room paying a bit of attention to the discussion about freeing up public data with a headphone in one ear so that I could keep up with the discussion happening downstairs.
And I get to keep a record of my participation. The chat I had with Clay after his talk has vanished, but every word I said to the team from Chinadialogue in our Skype chat last week is carefully preserved, including the moment when the clock chimed and I thought it was interference on the line.
Although they won't replace physical meetings, more and more of us are using online tools and services to keep up to date with our friends, share information about what we're up to or interested in and even to take part in events which we're not physically present at.
But travel takes time, costs money and can have an environmental impact that outweighs the benefits of actually "being there", so we want to make sure that the alternatives are as good as they can be.
And we can't assume that there is a single model for success, or that something that works in one context will be effective or even acceptable everywhere.
Clay Shirky was in London to talk about the ideas from his recently published book Here Comes Everybody, and in it he outlines what he sees as the social and technological factors that determine the success of a social tool, whether a service like email, a site like Facebook or a video-conferencing tool offering "virtual presence" at meetings.
For Shirky every successful social tool brings together a promise, a tool and a bargain - or rather, a plausible promise, an effective tool and an acceptable bargain with users.
The promise is the premise for a group's formation, the reason why people might sign up, join in or take part; the tool makes the necessary interaction possible; and the bargain is the deal between the provider and the user about what will ensue.
If you want to launch a new service then you need to find a way to bring the group of interested people into existence by promising them something, you need to find a tool that will do the job, and you need to keep your side of the bargain once the community is in existence - however ephemeral it may be.
Tools work in different ways, and they also build different levels of engagement between their users, from contribution (which Shirky calls "sharing") through cooperation to collaborative production, and ending with collective action. Wikipedia is a good example of cooperative creation; Linux of collective action.
We are still at a very early stage in our use of online tools for interaction and community-building, so we should not assume that any of today's models will persist or the big sites will be around in a few years time.
What is clear, however, is that the boundaries between the online and offline worlds are blurring as we put our hands through the looking-glass of the screen to shake hands with those on the other side, occasionally pulling them back through into what we still like to call "real life".
Bill Thompson is an independent journalist and regular commentator on the BBC World Service programme Digital Planet.