It does seem that they have changed their pricing and terms over time. I don't think you would really run afoul of their licensing so much as bump up against the limited number of users that are supported with the free package.
I can tell you that for this site I pay $100/year but I contacted them directly and managed to get a better deal that has much higher user limits than the normal $100 package. However, it may have been that they were willing to negotiate with me because I integrated their service into mojoPortal, so not sure how flexible they are with others.
To me, Facebook is just one provider and the real benefit of RPX is that they handle all the differences between different authentication providers and they keep up with security issues and API changes. I get many more site registrations based on Gmail or Yahoo users than on Facebook or Twitter, though I do get quite a few also using Facebook or Twitter.