We covered a bug that many of our readers tipped us on that saw the Twitter contact integration in Mango experiencing some difficulty. The problem being that individual contacts and groups were not displaying Twitter connectivity regardless to what settings/filters where chosen. While the Twitter feed is working, there's no way to view contacts who are also on Twitter.
A work around was available that saw users re-adding the Twitter account. The reason why this occurred was due to the social network modifying their API, which broke connectivity with Live. Joe Belfiore has confirmed, via Twitter, that the team has looked into the problem and the fix should reflect across the board within 24 hours. More details below from Microsoft Answers:
"We experienced a hiccup in our Twitter connection for contacts, but the connection is up and running again. Your contacts will reappear in the next 24 hours. Your Twitter feed and ability to post to Twitter remain otherwise unaffected. You may need to re-pin Twitter contacts to Start, re-add Twitter contacts to groups and re-link any Twitter contact links done manually. Sorry for the inconvenience."