Here is the outcome of my little research...
Best Twitter Client for Windows Mobile
PocketTwit
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Best Facebook Client for Windows Mobile
Offical Facebook Client by Microsoft
Best LinkedIn Client for Windows Mobile
There is no LinkedIn client for Windows Mobile yet at this moment.
Best Youtube Client for Windows Mobile
The official Youtube client
Best Picasa Client for Windows Mobile
No standalone app available yet. From a web browser you can visit m.google.com/photos to acccess the mobile picasa site.
I thought the new TweetDeck on iPhone/iTouch is pretty good.
Agree - PockeTwit is the BEST FREE app for winmo phones. I have been using it since it's build were in 40s, 50s.
I would love to have an app for Linked in on Windows mobile. XDA Devs anyone listening?