In my personal opinion, the #1 rule for the success of an online community is that members should not have to go somewhere they don’t already go in order to participate. Since that goal is usually quite challenging, the fallback is that if they do have to go somewhere else, there better be a significant quantity of links from where members are to where the online community is.
There are only so many places that people exist in the online space. Facebook, Google, maybe LinkedIn, some to Ning. After that it gets pretty fragmented. Along comes Twitter. Different people join for different reasons, but once they are there, the barriers between communities are negligable. If you are wondering what does he mean by “communities on Twitter”, then you should check out the hashtag communities that are starting to form and the tools which support them (Tweetchat, Twubs, wthashtag). A hashtag is just a keyword with a ‘#’ in front to denote a topic.
The intersection that I am talking about is when people post multiple hashtags in a tweet. Eg. “There is a great new list of Professional speakers on Twitter: http://bit.ly/ManhC #spkrchat #eventprofs” The speaker list is something that may be of interest to both speakers who might want to add themselves and event professionals who might want to book or at least follow certain speakers.
Now the members of both communities are aware of each other. If this cross-posting continues, there is likely some cross-interest between the communities and therefore some intersection of membership. Those who are members of one will find the second community very easily and have constant links to it put in front of them.
eg. If I am an event planner (member of #eventprofs) who books a lot of speakers, I am gaining awareness of this other community (#spkrchat) and become far more likely to join and participate.
If Twitter continues to grow and remains popular amongst regular users, watch for these communities to become quite important.