compared to what? they are two of the most successful tech companies built in the last decade
They really aren't. Or at least if they are it's a terrifying commentary on tech companies because neither of them have ever actually made money.
in terms of returns for investors/founders/early employees, yes they are, and twitter is undoubtedly in terms of reach/impact one of the most important tech companies in the last decade, whether they actually sustain moving forward is another question
So was pets.com at its time (and countless others). The time scale is different, but ... if twitter crashes and burns soon because it can't make any money, it will have left more impact on the history books, but ultimately will be remembered as "a facebook like also-ran, that only had a whats-on-your-mind line"