Maybe my information is not actual and I'll will be very happy if I was proven wrong on this, but IIRC there is no X acceleration (yet). Without X acceleration, you can't expect much "fluidity". From my experience with device based on Telechips 8902 SoC, the linux desktop without X acceleration was really sluggish. The ARM was only running at 540MHz, but still... As I'm not a facebook user, I don't know whether the facebook games are flash based. It might be a problem if they are, again IIRC the flash is not supported (yet) and I don't know whether it will be considering the recent statement from Adobe regarding ARM (un)support.
I'd like to stress that I'm not trying to bash the Raspi here, as a matter of fact I'm looking forward to owning one. If my information is incorrect, please correct me.