I am not sure who told you cricket is the fastest growing sport in the world.. It definitely is hugely popular in certain parts of the world..in the subcontinent..it is clearly the number one sport over football except if there is a World Cup going on..But otherwise..Cricket always takes precedence over European football leagues..
Its actually impossible to talk about decline or growth of cricket in general without taking into consideration the different formats and their different audiences.
However, having said that, there is a fair bit of fire in the argument that cricket is on the decline.
Matches earlier this year between Australia-Pakistan and Australia-West Indies were played with empty stands.
Test matches are relatively less popular in the subcontinent, very popular in Australia, England and South Africa(?)
What has happened is some teams have developed a quite deal while others have declined a quite deal. Two glaring examples are the growth of India and English test sides and the decline of Pakistan and West Indies as test sides.
So when Pakistan/West Indies plays Australia or England, it is a foregone conclusion that it is going to be a whitewhash.. Understandably, they are not very popular.
At the same time, you pit Australia against India or the Ashes for that matter...you will struggle to find a seat.
As for cricket spreading to other countries...while we do have teams like Afghanistan and Ireland coming up, it is still too early for them to compete at the international stage. We havent really had a major cricketing nation since the last 20 years which is not necessarily such a bad thing.
Cricket by its very nature is a niche sport unlike football, it goes on for 5 days in its highest format, it is always going to be more difficult for it to find new admirers. 9-10 test nations is not a bad thing by any means however with the advent of T20 cricket, it makes it easier for nations like USA to accept the sport and be a part of it.