It's many things...... but it's definitely not a surprise.
There are so many people with Facebook accounts, so many posts daily, it has huge internet penetration. The only thing surprising about it's integration is that it's taken this long to come.
Facebook has huge reach now. Nearly every blog, news site, forum, shop front has a Facebook login option. In the case of some apps and sites it's mandatory! I fully expect not to be able to open my car door without logging into Facebook soon.
But, for me at least, it's not that welcome. I does, however, look like it's 'under the bonnet' so you won't have the grinning F everywhere, just easy access to share and post from the activity menu, if that's your thing.