I dont know if someone already came with this idea or theres a plug in for it.
It would be great to create a User compatibility plugin in which acording to the user birthday, select the user Horoscope sign. And then when the user goes into another user profile, a small text would display:
Warning! You could make good frienship with this user acordign to you sing Capricorn and this user sign Scorpius
Or something like it.
I can provide with all the compatibility features, a simple sign to sign compatilibity.