I noticed that in the InternetAccounts folder in the System Library on my iMac's HD (late 2012) that there are a bunch of folders that contain ".iaplugins" from such unwanted companies such as Facebook, 126, 123, AOL, Google, LinkedIn, Flicker, Yahoo, and Twitter. I have no idea what these are for and have no desire to have my Mac associated with any of these "accounts" (there are a few for Apple accounts, such as iCloud, Mail, Notes, AddressBook, etc.). I assume I should leave the Apple plugins, but is it safe to delete the third-party plugins, and, if so, what is the best way?
These aren't third-party plugins, but actually first-party plugins (created by Apple) for third-party services. It is not recommended to delete these, as with anything in the /System directory, however you can if you wish, using Finder or Terminal (authentication will be required).