Or... It's about making the right product at the right time and reaching the right market. The problem (even if we consider the social one) that solved facebook

Or... It's about making the right product at the right time and reaching the right market. The problem (even if we consider the social one) that solved facebook wasn't that hard to solve. The timing and bootstrap / first users (universities) was the key. For Youtube the key timing was about bandwidth and for Google about technology/research (as well as a new search vs organize vision of the web).