Alternatives to RPX?

Answer 1

You can link the accounts in your own database.

Answer 2

if you need the adressbook importer of rpx --> you could also use http://openinviter.com/

(free, open source, php)

Answer 3

Yes, having same features as RPX have. You can use GIGYA for your requirements. Here is the link : http://www.gigya.com/