20-odd-years ago when I was but a small boy, I was already a keen spotter. I used to while away hours in the back garden with my trusty Argos 20x50 binoculars and a notepad. I can clearly remember fantasising about having my very own ATC terminal so I could identify anything going over.
As I got older my plane-spotting hobby fell by the wayside and I pretty much forgot about that sort of thing. Just recently, and now with my own little boy, I have started to get back into it. And what a pleasant surprise these live aircraft-tracking websites have been! Every time I use one I cannot help but marvel, "If only this was around when I was a little lad!"
Granted, the technology is still young and not without its problems, but it is still a significant leap forward. If somebody had suggested home live aircraft tracking 20 years ago, they would have been laughed at. Yet a great many of us now frequent these fantastic, and FREE, websites daily.
At the moment the technology relies on generous individuals uploading their SBS data to be shared with the rest of us, and this is why the technology is susceptible to Internet connection issues. Sometimes there may not even be any shares online to provide enough data for a functional service.
Personally I find that I pick up more on flightradar24 than on say, radarvirtuel. I tend to get most of the high-up stuff in near-real time. However stuff going into my local airport (MAN) is very hit-and-miss.
If you were really serious about tracking aircraft movements, you could invest in your own SBS box (or whatever is the current trend, I am not up-to-date) and then you wouldn't miss a thing. However, as has already been said, a great many aircraft, private and military in particular, do not transmit their position.