I downloaded my account data archive from Twitter, Facebook, Instagram, LinkedIn, Mastodon, and Flickr. The files below should give you a sense for what kind of contact details get included in each export. Basically I'm wondering to what degree you can look at a list of contacts and use it to keep in touch outside of that service.
If you see a note that says "We can make this file beautiful and searchable ..." that is something that GitHub added because the CSV files are not technically valid.