If it’s fit for purpose, buy it. I’ve had the 920 since last November and I am really happy with it. It runs really smoothly and all the apps that I need are already available. I came from iOS so I really don’t have a clue about Android. There’s some small things that absolutely do my head in, such as no rotation lock or volume profiles but otherwise I’m very happy with it.
The Whats App has just received a huge update which now means its working very reliably and as regards Facebook, there is two apps from Microsoft that I would recommend; Facebook or Facebook Beta.
If you get a Nokia Lumia 920 you will not be dealing with lag at all anymore, you will however become an expert in battery management. Not saying that the battery is poor or anything but its definitely something I am aware of. Largely because I can’t put the bloody thing down.