Most alumni events that I've been to start with unstructured cocktail hours. I've found that this leads to current students mainly talking amongst themselves. I wonder whether events that start with more structured activities first would work better. It would also add an incentive for people to show up on time.
It is difficult to facilitate meaningful interactions between alums and current students. On the one hand, organizers need to add enough structure to facilitate discussions between people with little in common, while on the other hand they need to leave room for unstructured time when people hit it off.
It's important that even those who are less outgoing come away with new relationships that they want to develop further. Assigning seats for a small portion of an event can help to introduce people who may not have met otherwise.