Regarding why there hasn't been a huge surge in guitar players; I would think it's because the guitar is already popular.
There are already millions of guitar players. People see the guitar everywhere. The guitar is very common - it's nothing new. So in this case, I don't think the internet is showing anyone anything they haven't already seen and heard on an almost daily basis. Mainstream music (almost all styles), TV, Movies, etc. have guitars all the time. The internet may be making it easier for people to learn guitar, but I wouldn't expect it to create a surge in interest.
With the uke though, it's not top-of-mind with most people. So, it seems that a person's first exposure to the uke could very well be the internet. They see a UOoGB video, or one of Jake's videos and think it's funny, cool, impressive, whatever. Maybe that leads them to look for more information, and maybe they want to buy one. That person may not have ever seen (or been reminded of) the ukulele had it not been for that video.
That's what happened with me. I played guitar for years. I knew about the uke, but was never interested in it. Never even picked one up. Then one day I was on Break.com or a similar site, and saw Jake's Gently Weeps video. That was impressive. I had no idea a uke could sound like that. I checked into them for a few weeks, but decided I didn't want to spend the money on one.
Fast forward another year or so, and Jake's video popped up again, and I was reminded about the uke. So, I started looked at other videos on YouTube. I'm not a big fan of Jake's style actually (he's a great player, just not my style), but because of the internet, and the seed planted by Jake's video, I was quickly and easily able to find lots of other styles that were more to my liking. After doing a bit of research, I went ahead and ordered a cheap Mahalo to see if I liked it.
Had I not seen Jake's video, or any of the other videos showing me other styles - all on the internet - I would probably not be playing the uke.
Jason