That's not a challenge, it's just common sense for *most* people. Facebook has too much functionality to bother fragmenting your own user experience across other services. Google is also looser about privacy than Facebook these days and I don't appreciate how they're trying to force people to join Google+ to use other services (those people who just want a YouTube channel won't actually use Google+ BTW, they just want to post their videos and troll videos on YouTube, just saying).
It's like having Facebook + Flickr +YouTube + Yelp + Foursquare + Google Talk + whatever when Facebook hosts Photos/Videos, has Places, has Check-In, has Messenger, etc. It's just much easier to do everything on Facebook, and trying to get people to move to Google+ as you cross post all your content between two services is not worth the energy.