I am a Linux geek and I am surrounded by geeks, but I also live in the real world: I may not see 99% of computers running Windows, it may be around 90% (approximate numbers pulled directly from my ass) so I still have a sane look of the computing world. And one of the aspects of this world that made me think was the omnipresence of WinRAR on the computers around, it is close to 99% of the desktops and laptops with Windows (again, anecdotal from Romania), to the point where all the computers I encounter have WinRAR - I usually uninstall it and put 7-Zip instead, able to open .rar, archives to open and create .zip, Free Software... When I happen to install a Windows, I put 7-Zip right from the start. And sometime it happens to see WinRar added back...
I gave this some thought, what's the reason for WinRAR's omnipresence and I concluded it the network effect: RAR was used a lot for warez, it was popular for its compression rate (a bit better than Zip) and the ability to make volumes (the warez was packed in floppy-disc sized volumes). So a lot of people needed at some point a .rar unarchiver, they went with the first hit from search engines: WinRAR, which once installed put itself as the default, making all future archives produced from that computer in .rar format. The archives were shared, resulting in other people needing and unachiver, other WinRAR installs, defaults and more files sent that way.
Today we have broadband connections, fast internet and large storage, most of the files we send are lossy compressed (jpeg, mp3) and it make little sense to compress them further (practically no space is saved) but it is still convenient, is way easier to send one file, the archive, instead of many - think for example how many clicks would be needed to attach them in a webmail interface.
Of course, it also helped WinRAR was nagware: a full functional copy could be downloaded at no cost, it (I don't know about today, never used it for years, I am just uninstalling it) used to have a nag window, asking to pay or press one more button (IIRC), so people were not pirating it and had no reason to feel bad.
Having in mind those thoughts about RAR and its network effect, I noticed this morning an article about how Live Messenger is censoring links, IM is another case of network effects, and here in my country Yahoo has a monopoly on that (I talked about it before). Still, there are a few dents in Yahoo's monopoly, made by the Facebook chat (we geeks with Jabber and/or GTalk are an insignificant minority).