The show was popular when it was one each week and even though it has been cancelled it recorded about ten million viewers each of the first two seasons. The third season saw a dramatic fall in viewers, but that might have been due to the fact that it was known the show was not coming back.
Just less than five million viewers saw the premiere of season three and that was half of who saw the premiere of the first two seasons. Ratings and the basic critical response is what networks are going to use to see how a show is doing, how it is received and if additional money is worth it to continue.
The Following Season 4 is not likely to happen, but as long as those that are familiar with the show keep bringing it up in interviews, the fans are going to keep the buzz going with their fingers crossed.