When you look at a member you can see the source of their signup, which can be something like Google, Facebook. a utm from the url.
However, there is no way to sort members by this info.
You can also see what page they signed for, and you can filter for this in the member page.
But neither of these are in the Member API.
Being able to filter and to get the data via API would allow sites to treat signups from different sources differently, and to measure things like conversion rates over time.