The internet as a whole, the growth of web based applications, and abundant mobile web access is what makes chromebooks so cool.
No single site will make much difference, it is the fact that most day to day tasks that used to require a big client computer can now be done just with web access, and that mobile web access has become ubiquitous.
I've been a Google+ user for a few weeks now, up until a few days ago, I was alone among all my friends so I hadn't got a real chance to use it fully. Now that they've opened invites out to a lot of people, I've got to play with it fully on my chromebook.
Everything works perfectly including video chat from what I can see. Pretty much everything Google related has been upgraded to work via HTML5 which helps chromebook performance.
Google+ does really serve as a slick merging of Twitter and Facebook functionality, I really like it thus far. Chrome has a ton of good extensions to do with Google+, and the Android App is pretty impressive as well.
It isnt much of a sales driver, its tough enough for people to move from phone to phone let alone moving over to a whole new social media. As for selling point for a product to have social media sell your product i dont think soo... i mean i could have google + on my pc...