Twitter embedded their product and users in hundreds (thousands?) of other applications. Without this coverage I doubt it would have been so successful.
Edit: I do recognise that there is a chicken / egg situation here. But I still stick with point.
Absolutely. Their reach would be downright tiny if we couldn't use Echofon on our iPhone, or tweet blog posts we find interesting with one click, with those pesky url-shorteners making it possible to share links.
The API.
Twitter embedded their product and users in hundreds (thousands?) of other applications. Without this coverage I doubt it would have been so successful.
Edit: I do recognise that there is a chicken / egg situation here. But I still stick with point.