This Slashdot comment sums it up:
this is finally a thing that really made me thinking of just closing my Facebook account. Not just opt-out from the new features again and again, since they just seem to always be more and more privacy intrusive.
This doesn’t use any kind of login button but shares the data automatically to a website when you visit it, so they instantly know who you are along with other data. IP data is still anonymous enough (from the view point of website operator – they don’t know who you are without going through police with a valid reason), but now the third party website owners have your name and other details without you never giving them those.
And just wait until every website will start to require you to use this. A good path for throwing all the anonymous cowards off the net and to get everyone comment and visit websites under their real name.