Spiderfoot - https://github.com/smicallef/spiderfoot/blob/master/modules/...
Resource used by Spiderfoot to do account lookups - https://raw.githubusercontent.com/WebBreacher/WhatsMyName/ma...
The linked tool uses a common approach which involves retrieving the account page in question and looking at the HTTP status code and text on the page, determine whether the username is in use. This makes it easy to expand it to a large number of sites as long as they have web-based profile pages, but the approach is error prone and not fully accurate (false positives and negatives occur).
socialscan uses a different method of checking (mimicking verification calls) that has to be implemented manually for each platform, but the accuracy of results is guaranteed. Of course this doesn’t work on all sites, but it isn’t dependent on platforms having public profile pages and can be expanded to email addresses as well.