Today Facebook proposed me to add my lunar calendar birthday so that my friends can know on which day of the gregorian calendar it falls.
I don't know if I saw this feature because I live in an Asian country, but I found it quite a good idea.
Actually, it could be helpful for many people because the lunar calendar is ever changing in relation to the gregorian calendar. Meaning, you cannot say "my lunar birthday falls on January 12" and be done with it.
It changes every year.
In Korea, people still celebrate their parents birthday based on the lunar calendar, and all I can say is that younger generations find it a bit confusing. So much so that they have to ask their parents "on which day is your birthday this year?".
So basically, if Facebook tells it to you, it might prevent some awkward situations (provided the parents are on Facebook too).