I think that Google is not magically adding their +1's to PageRank whilst not doing the same for Facebook likes etc. But rather that google plus has been designed by Googlers for Google's engine. As such a post on a plus account will attract percentage-wise far more +1s (same way a facebook post will get more likes).
And so if you compare two posts, one on facebook, one on plus.google, and they both have the same number of backlinks (likes/+1s) then the plus post will be better SEO friendly and so outrank.
The fact it out ranks so much suggests that something screwy is going on - either there is a lot of cash left on the table for SEO-friendly pages, or that something like being on the google root domain helps in ways that have not been corrected for.
I would doubt if Google is out and out cheating. But a lawsuit would force them to say how facebook could up its pages...
Edit: I know I'm not supposed to but really - why the down votes? I would be very surprised if Google is trying to kill the golden goose.