Do I seek permission from Facebook,Twitter,Yahoo to do this (Please view details)?

[just "JR"]

No legalities involved in this one...
You "just need" to use OAuth or OpenID systems and the relative SN's APIs.
BEWARE: it is DEFINITELY NOT for beginners!
You need a very solid experience in Php and the use of API to even get close...
Google OAuth and OpenID. Masses of details, masses of scripts, masses of documentation, masses of complications...
OAuth is free, while OpenID requires a paid subscription.

(With 18 years experience in web development, it took me three weeks to get through...)
(I now offer it to my clients, but the fee is 1000€ for the set-up, while I can make you a full e-commerce for the same price!).