Are questions about Twitter (or other web services) on-topic for Superuser?
If not, which StackOverflow-based forum should one use?
If the questions are related to the programming APIs then the best place would be SO.
If it is about twitter in general, you need to use the twitter support sites, as this is outside of the scope of S[OFU]. Twitter falls under the same category as Facebook discussed here
Not all computer-related questions are covered by S[OFU] sites, and I think this topic falls in the set of things not covered (except, as Diago pointed out, Twitter programming questions). Questions about video games are another good example.
It's quite arbitrary, but nobody ever claimed that all things computer-related should be covered on one of the sites.