The issue is obviously one of semantics but your concept of dependence is not the widely accepted concept. Dependence means completely reliant on - no substitutions would do. The thing you're dependent on is a requirement, not a preference. For example, you are dependent on food. Remove food from the equation and you will perish. You are not dependent on Chick-fil-A.
Twitpic is dependent on twitter. There are no substitutions that they can make and still survive. They can't port to a service comparable to twitter as there are none.
EDIT: Whether or not users are dependent on twitter is up for date. I imagine it depends on their use case.
> Dependence means completely reliant on - no substitutions would do.
I disagree with that. Package managers have concepts like optional dependencies or alternatives (only 1 of multiple options required). Anyway, you seem to be speaking of dependencies in a technical sense - build dependencies, package dependencies, etc. In a more general sense it is quite normal to say something like "I depend on my bike to get to work" even if I could take the subway or use Uber, or "I depend on YouTube for my daily dose of cat videos" even though you wouldn't really die without them.
Twitpic is dependent on twitter. There are no substitutions that they can make and still survive. They can't port to a service comparable to twitter as there are none.
EDIT: Whether or not users are dependent on twitter is up for date. I imagine it depends on their use case.