Soon, and as early as today, Gmail users will notice a change. After answering a prompt, their email "compose" windows will look very different: Gone is the frame that takes over the inbox and most of the screen, replaced with a smaller, stripped-down window in the lower right corner. Unless you open the text formatting bar by clicking the small "A" icon, the window looks almost exactly like a chat box:
As a UX change, this makes a lot of sense: Users don't have to leave the inbox to write an e-mail, and they can now edit more than one message at a time. But, like almost every recent change to Google's services, this is part of a bigger plan to remake Google in the image of Facebook. Google's original and current business model, based on targeted click ads, is becoming less viable. They need a new one, and Google's cofounders have decided that means social.
Most of Google's "social" changes have come under the banner of Google+, its social network. But the transformation, as it is being executed, is to be complete: Google+ has already peeked its head into Gmail, Search, News, and Drive (Docs). This new addition to Gmail is less about changing how it works than how it feels; if Google+ isn't going to be users' always-open social front page, Gmail, with the social network's red notifications, beckoning from the navigation bar, is better than nothing.
Think of it this way: Gmail and Facebook are both operating messaging services, each with delayed (messages and e-mail) and real-time (Google Talk and Facebook Chat) modes. Both services let you send either kind of message from the home page — the only difference is that, for now, Facebook's home page is a social home page and Google's is, well, Gmail.
If Google+ somehow eventually succeeds, or if Google promotes it hard enough, that will cease to be true: Gmail will be a feature of a social network, not a standalone service.