Building out what is forming in my mind would take me some time, but if I think there's a decent chance it'll (eventually) be widely used, it'll be worth it.
Hosting isn't an issue & I don't envision making any money from it (to address RIP's post), at least not in the near term. If there were ever enough monthly visitors to make it attractive to advertisers I wouldn't be opposed (of course), but that's not the purpose...
Setting up a structure that could both handle any type of event, including the local social events, and is easily searchable / filterable to allow users to find exactly what they're looking for would be the key to making it useful. Allowing export of events to personal calendars (existing Google, Yahoo!, etc.) should be part of it, and potentially the ability for users to create a login & store / bookmark their own list(s) of only the types of events in which they are interested would be nice (and fairly simple) as well.
As for getting the info into the database, that would be up to those who are interested in seeing the events listed. Distributing the ability to enter information away from a centralized admin would be key - otherwise it
would be a full time job & I've already got a couple of those...
Marketing & SEO would take some doing, but a lot of it would come down to creating a system that is easy to use & contains information a large number of people find useful. Implement that and it'll grow over time via inbound links & word of mouth.
It might be fun to give it a shot - even more fun if it gets used...
![Cool 8-)](./images/smilies/icon_cool.gif)