A number of approaches to social events have been tried with varying success.
Conference meal
Undoubtedly the most successful social event has been the conference meal, provided at both Connect #1 and #2. In both cases this has effectively been a standing buffet organized at a convenient bar. One "problem" in describing this event is not to have delegates expecting a full "conference dinner" as at, say, Open Repositories! The pre-conference survey should be used to elicit any special dietary needs and we should say only that we will try to accommodate them; vegetarian and vegan are usually easy - beyond that it gets trickier.
For HC2015 we made this optional and charged for it separately; that worked too with about 70% attendance. Again it was a standing buffet and there were some comments about it being dubious value for money (especially if you were teetotal and couldn't take advantage of the two free drinks).
Beer bus
For both Connect #1 and #2, the locals (independently of the Connect organizers) organized a "beer-bus" to tour the local micro breweries. At Connect #2, there was criticism that the trip was too expensive (even though the price included a t-shirt) and that the emphasis on beer was bad. (Some people don't drink beer, some people don't drink alcohol.)
Sign-up dinners
For both Connect #1 and #2, we organised on-line sign-up sheets for dinner groups - the idea being that newcomers could maybe team up with each other and with some "old hands" so that they weren't left on their own in an evening wondering what to do. These have been of limited success. They perhaps need coordinating further in advance than we did for #2 and perhaps they are best not held the night before the workshop day when people may still be arriving at the airport. It has been noted that Code4Lib assigns hosts...
Other?
Other suggestions for social events include other kinds of "bus", a games evening, ...