The Policy configuration is looking fine. The only, possibility which might block "twitter" traffic through the PAN firewall, if App-ID is unable to identify the traffic correctly and twitter traffic is identified as "Unknown-TCP". I had a similar issue, where "twitter" traffic was unable to catch by App-ID ( Application Identification).
Hence, could you please create a test policy for one test machine and allow Unknown-TCP and let us know the result.
Could you please let me know, what "Application-Threat" version is running on your firewall. In my test box, i have 437-2224 installed and twitter is working just fine. If your PAN is not running into the latest one, could you please install the latest one and check.
You may try to do a NSLOOKUP on one of the testing PC, see what is the Twitter server IP address that it is trying to access.
After that, in the GUI --> Traffic log, you may use filter like ( addr.src in IP_ADD_OF_THE_TESTING_PC ) and ( addr.dst in IP_ADD_OF_THE_TWITTER_SERVER ) to check the security policy that the traffic hitting.
Also you can check the real time session in the CLI by using 'show session all filter source IP_ADD_OF_THE_TESTING_PC destination IP_ADD_OF_THE_TWITTER_SERVER'.
There are many possible reasons, which could cause this issue. As mzh said, you could check the real time session in the CLI by using 'show session all filter source IP_ADD_OF_THE_TESTING_PC destination IP_ADD_OF_THE_TWITTER_SERVER'. Also, grab the session ID from there and apply a CLI command PAN> show session id XYZ >>>>>>>> to get detailed information about that session, i.e NAT rule, security rule, ingress/egress interface etc.
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