Hi,
I would create this in html5 with native apis included for each platform such as mac, windows, ios etc.
Then I would create the server to which the app will connect to pull the user info, voice streams etc. This will be done via api, the app itself will include the functions but will connect to the api on the server in order to make it secure. When the user connects via the app, the api will return the users friends list, chats, account balance stuff like that.
Will this be a free app or a commercial app for desktops and mobile devices?
Regards