That's not how it works, just look at MySpace. Even sites with a lot of market share are sometimes overtaken by a competitor.
The best alternative to Facebook in a fragmented market like Europe is federation across multiple social "hosting" sites with a regional focus, based on standards that are either in place already or being worked on. This can exploit even stronger platform effects than a conventional one-size-fits-all solution like Facebook.
Google and YT are in a different position since they're actually solving hard resource problems (Video streaming at scale is hard; building a usable crawl of the Web is also hard); much of their success comes from addressing these in a way that is compelling for users.
