Well everyone used to think the earth was flat, so I definitely think their numbers are on the decline. As far as other conspiracy theorists, I really think the reason they seem to be increasing is because of social media. We never used to hear about them or from them - now they have a platform to get their message out.
The solution may be in the messenger. Scientists aren't always the best ones to try to convince skeptics. I saw a cool video about a scientist who convinced the Pope that climate change was real and worth doing something about (Scientist gets the message out). He is someone that many trust and believe in and will listen too, and it made a difference. You have to convince a leader in their community to believe and he or she will do the rest.
But don't waste your time on the Flat Earthers...they're just stupid.