I am thinking about the difference between email, QQ/MSN, twitter,blog, Facebook/Google+, Yelp/Dianping, Douban, Google group, web forum and mail group. It is possible to merge them together? Actually it is all about how to distribute information.
The first step is to identify who you are. In email, we use email address; in QQ/MSN, we use QQ number/Windows live ID; in web forum, we use user name. One thing need to emphasize, email is wonderful. Since we could talk with other in different world. But for others, you could only talk with one in same world. I think we need to talk with other worlds since it is impossible and not safe to only have one identify system. But in QQ/MSN world, it seems we lost it. But web forum/web site, open ID is a good solution, but seems still long way to work.
Then you could decide who could read this information? Public or some groups or just one of your friends. Facebook did first step for this, and Google follow next with circle. But still, both Google and Facebook could only identify the guy in their world and there is no way to control access right based on email, QQ/MSN and mail group address. Maybe it is too complicated to implement and for user to decide. We need some break through on it, especially for email which nearly everyone have.
Third is to notification. How should you know which information need to read and which could ignore, especially the information is huge. We need some way to control the important level of information. Generally, we will read every email/QQ/MSN information and decide if to take action. But for web forum, it seems impossible, sometime we don't do that. This is also similar in Facebook. I think it is most important to develop some systems to solve this problem. WE COULD NOT READ EVERY INFORMATION!
Finally is how to find the information. There is so many information and how do you find them? Forum/group are a better way to organize information into different category. Facebook and Google+ is trying to give information weight based on how many people like it.
From this point of view, email,QQ/MSN is more similar which is distributed based on single one; twitter/blog is distributed to public by default; Facebook/Gooogle+ and mail group are distributed on groups; Forum/group is distributed not on identify, but on subject.