There's still a risk to Facebook that they'll come to be seen as stodgy as new generations come online. But given how well they've been conscious of, and managed, the risks of trends passing them by so far, I don't think it's a major threat.
In many ways Facebook's end-user lock-in is much stronger than even Google's lock on the search-user -- the folks who are now populating Facebook do not flitter to every new thing online, as a group, pulling all their relationships. (Google's economic lock on the advertising-clearinghouse side is still stronger.)
In many ways Facebook's end-user lock-in is much stronger than even Google's lock on the search-user -- the folks who are now populating Facebook do not flitter to every new thing online, as a group, pulling all their relationships. (Google's economic lock on the advertising-clearinghouse side is still stronger.)