I recently attended a reunion event for a company I worked for a long time ago. It's one of those big companies that has several decentralized divisions, and people would often move between them, resulting in some very intertwined networks. Having worked at three of the divisions, it was fun for me to see how many of us knew the same people, never knowing that we had them in common.
We all have access to networking tools like LinkedIn (and this was definitely more of a LinkedIn crowd than a Facebook crowd), and I had online connections with many of the people I saw there. But there was nothing like getting a group of former coworkers together to nostalgically remember the old times, and catch up on the new times. Something else happens that you can't quite capture online. Coworkers get a good sense of who you are professionally. But this view somehow stays frozen in time. When you catch up in person, you have a chance to readjust your mental image of what this person does. Of course some things never change, like basic fundamental values. The people who were trustworthy will always be that way. But along the way we all collect new skills, and it's fun to see how we have grown.
So we reconnect again, and hopefully use the online tools to facilitate the maintenance of these connections. But it's just not the same as the face-to-face. Everyone approaches these events with a bit of trepidation. My suggestion is that you should always go to them if you have a chance. If nothing else, it's good for the ego. You realize that you had many more fans than you ever knew at the time.