Comparing the Tweet, +1 and Facebook buttons next to each other, I can easily see how people just don't understand what +1 means. The button graphic has a colored ribbon at the top with the label '+1'. Next to it, I see the number of the total clicks. However, there is no explanation what it means. Compare that to the Facebook button. It says "Recommend", and "60 people recommended this".
'Recommend' is a clear call to action. '+1' requires a lot of interpretation and prior knowledge. It's the same issue G+ faces with huddles, sparks, hangouts and circles. By not adopting everyday language, features become mysterious.