Seconded. Compete's numbers are just made up. I know for sure they are off by more than an order of magnitude for reddit (or at least were a year ago).
Compete, Alexa, etc. they're all tracking people who don't understand technology.
That would mean that G+ is even stronger as it seems mainly popular with the tech crowd.
Specifically, those that have them installed are more likely to be non-tech users than those without.
That's my guess with 0 data to back it up (but plenty of useless anecdotes!)
The comparing reddit/digg on compete doesn't make much sense because of the digg button penetration. Just like comparing linkedin to google plus is stupid because many google properties make requests to plus.google.com.
Compete is a valuable tool, you just need to learn how to use it appropriately. Learn when it is reasonable to compare sites using it.