I'm setting up a network in which individuals see profile information of one friend, switched with that of another friend; such as likes, political views, etc.. Similarly for twitter, would see tweets tweeted by people who they follow. This would be editing only web interfaces of the two services.
I would like to have a transparent proxy on the network cache and serve the mangled pages; is this possible or even the correct way to attempt this? Thank you.
Since both Facebook and Twitter use HTTPS for their default connections, the correct term for what you're proposing is a Man-in-the-Middle attack. To do this you would need to do the following:
1) Deliberately misconfigure your proxy server to intercept HTTPS traffic.
2) Intercept the SSL/TLS handshake to get the session keys for each encrypted session.
There are commercial products which do this and are usually sold to businesses or government departments which need to monitor the activities of their staff, or who just want to.
A similar proposal was made around five years or so ago in Australia as part of the proposed Internet censorship regime. I wrote a report on that a year or so later and you might find it useful (PDF). Pages 6 to 10 deal with the part you're interested in, the technical methods by which it could be done.
Needless to say, I recommend against deliberately intercepting and interfering with your network users' secure communications. There would, after all, be no practical difference between interfering with their social media accounts and interfering with their bank accounts.
