Facebook. I was convinced this was a terrible idea until about 2007 or so, despite being on it since 2004. Still think it's kinda a terrible idea, but a billion people disagree with me. (Or perhaps it's more like a billion people agree with me but use it anyway.)
I also thought it was going to be a fad. What is interesting is that everything people use Facebook for day-to-day existed back in 2005. We used email for sharing life updates, various tools for photo sharing, AOL Instant Messenger for chatting, mailing lists for groups, email and evite for organizing events, etc. The novelty of Facebook was profile browsing and poking each other. And if Facebook had just remained that it would have been a fad. What is impressive about Facebook, is how they managed to little-by-little get everyone on their network, and little-by-little beat out the tools people were using before Facebook.
Interesting, now, in 2015, Facebook is a worse tool for me than it was in 2009. Any tool that my boss is on, my aunt is on, my old high school classmates is on, etc, has lost any coherence to it. I now have to obey standard internet security: anything posted on the internet or on Facebook in my real name has to be treated like it is on my resume. Because, de facto, it is.
I solved that by being one of the few people who have never used the site or made a profile, even though I could have done so back when it required a .edu email address.
I have a different experience. I don't know a single person who used to send life updates with email, or engage in mailing lists. At least to this individual, it was very obvious why Facebook was going to be a success when it arrived, it had all the basic 'tools' you needed to communicate and stay in contact with family and friends.
I think this is a very good example of why Facebook succeeded. Everything Facebook does was done (oftentimes better) by a niche tool with a niche audience of early adopters before. Facebook managed to cross the chasm with all of them and unify them together into a single social networking experience. It brought all these tools that were in use by small populations to the mainstream audience.
