'Followers' and efficiency

Pretty simple and easy to do with full normalisation. If you have a table of users, each with a unique ID, you would have a TABLE_FOLLOWERS table with the columns, USERID and FOLLOWERID which would describe all the followers for each user as a one to one to many relationship.

Even with millions of assosciations on a half decent database server this will perform well and fast as long as you are using a good database (IE, not MS-Access).

The model is fairly simple. The problem is in the size of the Subscription table; if there are 1 million users, and each subscribes to 1000, then the Subscription table has 1 billion rows.