It's not the Internet - few people in the world know of this forum. It serves a good role in helping those recovery from cult abuse, but it is not what is causing numbers to drop.
The reality is that people in the more modern West are less religious, less inclined to look to the Bible, but will sooner try medicine. The USA is somewhat an exception, with it's strong Bible belt, and people far more emotional about god - Europe is very different! For example, most Britons are somewhat atheist, so all those American presentations in Our Kingdom Ministry never worked, for example, you ask someone "Do you pray the Lord's Prayer?", and they ask what that is; children grow up with no knowledge of what is in the Bible. No one goes to church, they are becoming nightclubs or hotels. If a referendum was carried out, I expect people would happily abolish religion; Islamic terrorists are not currently helping with the image of religion.
The result then is that people are less inclined to religion, but instead turn to things the Watchtower warns of: vices like drink, drugs, lots of sex, therapists, university, and plenty of materialism. This is the competition! All of this distances them from the organisation more so, as there is not religious basis to prey on. China is also athiest, but the people have no recent painful history of religion, as does Europe, but have been so shielded of religion, so naive, that the curiosity makes them easy victims as JWs leap in and show them what the Bible actually says.
On the other hand, much of Africa is poor, where people have more of a respect for the Bible. They are less educated, so easier to take advantange of and mislead, and they don't have the distractions of materialism, so other than to get drunk, the Bible is a great hope for the future, a dream to cling to.
However, there are anomalies. Despite somehow gaining create progress in Japan despite the Shinto religion, the JWs have had little success agaisnt the strength of the Muslim religion, it being a fascist regime that dominates people. There has also been little progress agaisnt Hindiusm.