Is it possible to design a twitter like DB using SQL server? a DB that will ensure high scalability and fast queries.
I am building a .NET platform that requires a similar model like twitter (User, Follower, Tweet) and looking into what will fit best in terms of fast queries and scalability.
Will it be possible using a relational DB or is a graph db much better?
SQL Server will most certainly be able to handle any load that you have. SQL Azure supports databases up to 150GB (though I hear you can get more if you ask). With Azure SQL Federation, you can scale out multiple databases on hundreds of nodes around the world.
As for a relational database like SQL Server, or the "NoSQL" variants like Azure Table Storage, it depends on your needs and how structured your data is. Given you'll probably do a lot of joins, querying for followers of users, tweets that someone should see, etc. you're best bet is to go with a relational db. Even Facebook still uses MySQL, so you're not exactly in bad company with using a relational db.