Timing. People are actually using Genius - the news stories I found indicated a userbase in the millions.

With consumer tech startups particularly, it's oftentimes simply a matter of the public being "ready" for a particular idea. Facebook was far from the first social network; I was using LiveJournal in 2002, sites like EZBoard and Xanga came out around 1999, and before then there were all sorts of hosted forums like UBB (and UseNet, of course). But what made Facebook special was that they launched in a place (Harvard) and time (2004) that was ready for them, and then expanded quickly across the world. Their tech was trivial at the time; I could've coded up Facebook as it existed in Sept 2004 (when I discovered it) in a couple of weeks. But I wasn't the one who actually did it and got people to use it.