Once, we were having a conversation with our computer science professor about Wireshark and he told us, how he previously used it in a class and even saw facebook chats of some of his students. As I know, facebook is encrypted, so does anybody have an idea how he was able to do that?
Yes. Prior to the release of the tool FireSheep, Facebook, LinkedIn, Twitter, and other prominent social media platforms did not support TLS/SSL for all connections. In fact, companies generally claimed that the processing overhead would be too high, limiting their ability to serve customers effectively.
When FireSheep was released, most major social networking providers completely switched to TLS, or at least made it available for all requests, within weeks. Your professor likely did his demonstration before this change was made.
If you're unfamiliar with FireSheep, it was a Firefox browser plugin that allowed you to automate the collection of other user's session IDs from network traffic (via a network sniffer), allowing you to instantly impersonate any user (whose network data you could see) on major social media platforms.