Facebook fears Google an Google have not even flinch in the social circle yet.
Except a lot of Facebook users are Gmail users an Youtube users an google search users.
People still use them an there use is about equal or just below that of Facebook.
If Google can design a easy way for people to use there network connect there friends, then very quickly I imagine saying why do I have to go to a other site when my friends on Youtube, I can use my gmail address, an I can even phone, video conversations an instant message my friends, why watching Youtube videos together.
If Google can get this to work, design a easy to use interface that seamlessly with there products, this got a chance to be a huge hit an people could well start using with out even reliesing they using it, an once they are suck in they could well forget facebook.
Will this happen over night nope but if anyone can do it Google.
In many area Google services exceeds Facebook by an order of magitude in functuality. There a reason Facebook developer team went into lock down mode, it because they fear Google an what they have the capabilities of doing.