One possible fix for this is to reinstall/repair Windows Messenger completely. Perhaps, improper installation might have caused components of WLM to malfunction and show the wrong offline or online status of contacts.
To repair Windows Messenger , go to Control Panel > Programs > Programs and Features. Right click Windows Live Essentials 2011, hit Uninstall/Change. Click Repair all Windows Live programs and complete the process.
Restart WLM and the contacts would (hopefully!) be online now.