Wow, this is the big problem and challenge. That's the reason some online communities also have their offline events.
Several years ago, I joined and cofounded several online non-profit communities. For example, the TEDtoChina community which supports the TEDx local communities around in China. Also, Creative Commons China, etc.
This type of online communities has a clear vision and online activities, they have their brand, sometimes they adopted open brand licenses from global organizations such as TED. They worked like a non-profit organization, but an online group.
Of cause, this group of people are geeks and change makers. They are not majority. They can collaborate with each other.
As a member of this group, I often were asked about this type of online collaborate by mass media reporters. They just couldn't understand online collaborations.
Finally, I accepted that there are two worlds.