For Facebook, only business pages that have no access restrictions can be tagged (personal profiles cannot be tagged).

For Instagram, any page can be tagged but there is no autocomplete/autofill feature.

For LinkedIn, only companies can be tagged. 

For Pinterest, you should be able to tag any profile, but no autocomplete/autofill feature.

For GBP, tagging is not available.

To tag, type "@" and then the profile's or business page's name. When tagging someone on Facebook or LinkedIn, you can select a name from the list that appears with the autofill feature:

Please keep in mind that we don't currently have a way to tag profiles in pictures.