Merging personally identifiable data with social profiles requires data collection obligations to be observed.
In order to comply with privacy laws and social media channels' terms of service (e.g. Twitter), you need to have the express opt-in consent of individuals or another appropriate legal basis which permits such data merging, in order to lawfully merge their personal data in Audience.
Per Twitter Developer Policy (Effective November 3, 2017), Section 1, C, 2, you shall not “… associate the Twitter Content with any person, household, device, browser, or other individual identifier, unless you or the entity on whose sole behalf you make such an association do so (a) with the express opt-in consent of the applicable individual; or (b) based solely on publicly available data and/or data provided directly by the applicable individual that the individual would reasonably expect to be used for that purpose.“
You will be asked to confirm you have this permission when uploading and merging data into Audience. For example, when importing a csv file or choosing to merge individual people's profiles.
If you do not have the necessary permission, you can still upload data to Audience. It just can’t be merged.