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Risks: the site/service knows you have a social media account, and whatever data you allow them to access from that account when you grant permissions

Benefits: the login is being managed by a 3rd party, so any breaches associated with the site in question are not likely to give away [useful] login credentials (since 3rd party authentication is token-based through the auth API)

I use 3rd party authentication a lot – but almost exclusively on sites I don't care about (and with an auth option of an account I don't care much about)