data slurp
I do not use FB (obviously!)
But its my understanding these style of social media try to get a user to import their contacts e.g. phone contacts
I would be happy if this is not the case any more with GDPR, I know it used to be.
e.g. many family members have my contact details & visa versa (as obviously may want to contact each other for reasons from catchup chat through to family emergency depending)
I'm obviously happy with family members having those details but I'm MASSIVELY not happy about that data subsequently getting slurped by FB, Twitter, etc. as no way have I consented to that. Especially as numpty relative may well have added my birthday to phone contact details (as they obviously know my birthday & then get reminders) and so FB could get that and other stuff they may have recklessly added about me.
So, even though I never use my real birthday anywhere (bar legally required e.g. bank, doctor) there's a chance FB could get my real birthday (useful for impersonation attacks) & even more horrendous details (some phone contact apps store relationships, so via numpty family members FB could potentially get standard security questions such as mothers maiden name (stupid question anyway, lots of kids of unmarried mothers where maiden name = current name)