That's a big question.
But just to get you started you shuold think on how you would like to implement the data model.
Since socail networks tend to have mass amount of data i would personlly choose the NoSQL database rather thant the SQL Azure db.
You can use azrure table storage or the upcoming
hadoop db for azure.
Regards
Shay
Blogging on www.codeproject.com/Members/ShayYannay & www.questcloudtools.com