Are there any restrictions to tagging on Social Media Poster?

For Twitter, you should be able to tag any profile.

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 Twitter, 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.