Peer-to-Peet (P2P) overhauled the Internet medium. No longer did you have a single point of failure, you could rely on on multiple peers to fulfill your data requirements.
Site-to-Site (S2S) is analagous to web services with the emphasis being website-to-website data fulfillment.
A website presents a finished website. Before they used to link to content on other sites. Users would follow the link to get the content and you'd lose them. Web services (S2S) provide content for that originating website so that the users don't have to leave the site for other pastures.
The good side: you can keep your traffic. The bad side: someone else loses hits but not the bandwidth. There are remedies to the formula.