After searching the meta stackoverflow, apparently this tag "untagged" is used as a temporary state
What causes a question to be tagged as [untagged]?
Which means that these questions should be dealt with.
Other related questions on meta:
Review task for untagged questions
Do not allow users to tag questions with "untagged"
Around the internet there are several social media and online reference managers that are more of less of interest to academics: Google Scholar, Academia.edu, ResearchGate, Mendeley, LinkedIn, Facebook etc.
At the moment, among them, only Google Scholar and Facebook have specific tags; the others have been divided between the more general tags social-media ...