That's a little uncharitable. I joined Facebook fairly early on (when it first launched for college students), and it was the first social networking app of any sort to even hold my attention. Friendster came and went without me ever visiting, so did MySpace... We may not like all that FB is doing, but IMHO it was a lot more than a me-too, even in the beginning.
Turns out, those "features" weren't actually good things. And when MySpace turned into a spam infested, buggy, always down piece of crap we all turned to Facebook. Honestly I think Facebook blew up in that time because MySpace dropped the ball. I don't remember anything compelling about Facebook at the time when the bulk of my group switched, other than that it didn't break so much.
Yes, but lots of people wanted to be like Yahoo. I think the whole portal idea came from sites wanting to be like them.
Facebook and Google usurped their predecessors from out of nowhere. They both wouldn't exist if their founders said to themselves, "I'm not going to try to overtake Yahoo/MySpace -- they're unkillable."
I think Facebook and Google did exactly that. Facebook was just another "me-too" social networking site at first, targeted at a limited audience; I doubt that right from the start they thought they could be the #1 site on the internet, or even the #1 social site. And AFAICT, Google's initial goal was to provide a better search, not to supplant all other search engines.
In fact, in my opinion, what they have in common is that they both didn't aim for the top at first; rather, they carved out a niche, then worked from there.