I'll try to post a precise information about the website.
1. Website is about posting short messages like "twitter" by logged in users.
2. Users can login using facebook, twitter or normal account (registration on wordpress).
My requirement is to create a plugin that:
Posts the status as wall post to a somebody else's facebook page (page ID available). Facebook app reference will be provided, and the user will be a user who logged in to site using facebook. So If I am a user and I logged in to site using facebook account, My name will be mentioned while posting the message to the wall.
Post the status as tweet to somebody else twitter timeline if user is logged in using Twitter account.
If user is logged in using normal wordpress account then, use site's account and post message on both twitter and facebook.
Let me know if you have any questions.