Recently I’ve been thinking of services like Google+ and Chatter as flattened email. A post on one of those networks is quite similar to an email that you send-to-all, and to replicate email in Google+, you can simply make a post private between you and another person. The differences, then, lie in your interface to the conversation. This interface has profound effects – just think if you had to click the subject of every post on Google+ to open it for reading, then click back out of that post to see the stream again. Would that change how you used it?Email shows you the tip of each iceberg, whereas Google+ flattens them out and shows you the whole chunk at once.
We could say that Google+ and email are merely different interfaces to the same core features. Email is an efficient way to get an overview of all of your active or recent conversations, since it hides most of the content. Google+, on the other hand, splays it all out in front of you, which makes access to threads from further in the past more difficult, but makes interacting with recent content much easier.
Flattened-email products, which seem to be on the rise, tend to focus on the now, and less so on the ease of finding a conversation from the past, or storing it for reference. These problems will need to be dealt with quickly, or flattened-email will end up having usability problems that are different than, but equally maddening as, those that email has today.