How do I redirect my sites based on different referers?

This approach should work:

map $http_referer $redirect_referer {
    ~ ^https://l.facebook.com sub.example.com;
    ~ ^https://t.co sub2.example.com;
    default 0;
}

server {
    server_name example.com;

    if ($redirect_referer) {
        return 301 https://$redirect_referer$request_uri;
    }
    ...
}

First we define a map that assigns values to $redirect_referer based on HTTP Referer field contents.

Then in server block we check if we should redirect, then we do the 301 redirect to the destination. You might want to use 302 temporary redirect instead in this case, but you need to think about all the details.