As far as I can tell, the only way for me to gauge the efficacy of any signup method I have out there (text to join, web form, web button, etc) is to associate that button with a list. Some of those methods actually allow new signups to choose the list they are assigned to (a useful function.)
It would be really useful to tag contacts that joined through one method or the other. Then, we could also tailor welcome e-mails according to how they joined. (currently, welcome e-mails cannot be filtered by tags).
This would be really helpful in reducing the amount of list management we have to do.