Its a very socially orientated operating system, and allows you to view your friends facebook status' right from the home screen through "Live Tiles". You can even see whos online and chat to people on Facebook Chat and MSN Messenger right from your "Messages". And it groups them all together so you can see what you have sent and recieved from individuals.
There is even the location services built in so you can post quickly "im at the pub" or whatever you want and it will place it right on facebook for you. Obviously it has emails, and a great photo/camera suite and everything that you need from a mobile phone is there!
I have had several different types of phones in the past including blackberry, iPhone but the Windows Phone 7 (WP7) is truely amazing, it does everything that I could ever want from a mobile phone.
So if you are thinking of changing your mobile, its highly recommended to give Windows Phone 7 (WP7) a go!