Even on this prototype with an unfinished OS, Pocket-Lint claims the OS ran very smooth with none of the stuttering that current Windows Mobile users find on all but the most powerful devices. They have also taken a play from the Palm WebOS handbook in that Facebook contacts seamlessly appear in the address book. Microsoft has also done this on their desktop version of Outlook 2010 with LinkedIn, and Facebook and MySpace are to follow soon. Expect more, not less, social networking integration features to be on the final devices that ship this fall.
Windows Phone Thoughts also pointed me to a Twitter client that flew under the radar two months ago and reported on by Mashable. Bing map integration is included and overall, the client looks like a good fit with the WP7 interface. I cannot tell from the images if a built-in URL shortener is included, and even if it was, I'm not sure how you'd get the URL from the browser to the Twitter client sans a copy and paste function unless the Twitter client somehow hooked into Internet Explorer on the device. We'll have to wait and see how that develops this summer.
The Mashable page also indicated that Netflix, The Associated Press and Foursquare will have apps on WP7. That's four apps down, about 179,996 to go.