There is bit of difference between how the new Microsoft Lumia accounts will be named on Facebook and Twitter, if you have already noticed. Facebook profiles are named as Microsoft Lumia and Microsoft Lumia + region for regional accounts, while Twitter profiles only has Lumia or Lumia + regional for regional accounts in their names.
Windows account on both Facebook and Twitter will cater to the need of Windows Phone users using other devices than Lumia.
Nokia Facebook and Twitter accounts remain unchanged and will return to the real Nokia.