miss Google+ usage on mobile, through YouTube, Reader, Docs, News, and so on. The new Google+ integrations in Gmail will make these statistics completely irrelevant.
The trend is not new. Gmail has been the most plussified of Google's services. Gmail and Google+ have shared the same contacts and chat features. They both use Circles to organize contacts into groups. They show the same profile images next to people's messages. Gmail also has a People Widget which shows a person's contact information from their Google Profile as well as their most recent posts. You have recently been able to share images received in Gmail using Google+. And, you've been able to add people to circles right from within a Gmail notification.
Today, almost the entire Google+ social experience is coming to Gmail. If you have email notifications enabled, whenever someone shares a post with you or comments on your post, you will receive a message in your Gmail inbox. Without leaving that message, you can +1 the post, reshare it, comment on it, or read existing comments. If someone adds a comment, the message is updated in real time. The experience is completely synchronized between Gmail and Google+.
Tech bloggers have long speculated that, thanks to the rise of social networks, email will become obsolete. That hasn't happened, 'cause email stubbornly refuses to die. But it's no longer the preferred means of communication it once was. Rather than picking a winner, Google is merging the functionality of a social network (likes, comments, profile photos, etc.) with the security and simplicity of email.
Will this eventually become the face of all Gmail messages? I don't know. But I wouldn't be too surprised. The lines between Google+ and Gmail are blurring. And Google doesn't like keeping services with overlapping functionality.