I don't see my wife creating a google account so she can click +1 on a web page when there will be a facebook like button right next to the +1 button. Why would she? Her friends are on facebook.
This is a key point. Also, it's easy to "like" something if there is only one button. I feel I'm doing my friends a favor.
When there are multiple buttons, I feel like I'm doing work for the website operator. The only exception would be the case where the content is really so awesome that I feel the whole world needs to know about it.
My first thought after reading about how it works was "How can we consolidate all the vendors into a single 'like' button?"
Someone like add this may create one. There are already plenty of ways you can turn a like into a share on facebook, twitter, Tumblr etc instapaper's new social options is one
Her friends don't use Google?
Perhaps they do use google, but I suspect the number of people who actually use google as opposed to the number of people who have a google account is quite the gap. This is mostly because you simply don't need a google account to actually use google, while with facebook lacking an account essentially makes the site pointless.