As of "Facebook already does it", it may be true, but they don't show you all your friends events. For today I see only two, for example. I guess they show me only the most relevant.
The thing is, once you get some traction (and that's a big IF), you can do things like showing the most popular events in your city, even if none of your friends (nor you) has been invited. And with more volume of events, you can also create better recommendation algorithms.
Another use, which Crowdspottr already has and Fiestastic doesn't (nor Facebook, at least not "easily"), is seeing what events someone else has RSVPd to, which is cool to see where you friends are gonna go or what are they interested in. Someone could say this is great for stalkers, but I digress :P
Even if Facebook does some things with events, the good news (for Crowdspottr and me) is that for them events is just a part of their graph, and I don't see them putting a lot more effort or functionality on top of what they already have, when SO much can be done with all that data.
And yes, using other APIs as well like Foursquare is a good idea (Crowdspottr does it AFAIK) but I know personally I'd be kind of lazy to put in log in details for more than one service. That, and also Foursquare is way less ubiquitous than Facebook.
I think it might work better the other way around. You might need to show events outside my friends' knowledge to get any traction, because if my friends are already going to those events then I probably already know about them.
I agree that Facebook won't put as much effort into one feature as you could, but the fact is that people's time is limited and they are all on Facebook. So, you need to use FB to get traction. As a simple example, I don't even see a Like button on Fiestastic or Crowdspottr!! The first rule of social is sharing.
I guess you will soon add events other than just for today. It would also be nice to see the names of my friends who are going (right now you only show a thumbnail of their profile pic).
Getting traction for this app (and for Crowdspottr) might be nearly impossible though. Facebook already shows me what you're showing (on the left side menu, under Events, there's Friends' events). Plus the chances of me remembering to come back to your site when I want to go out next time are pretty low.
The only way you could do better than Facebook is if you aggregated events from other sites and made your own recommendations based on my past events. I think that's one thing which most sites seem to miss. If you know the bands, festivals, parties I've been to you should be able to tell me about other similar ones. Hook into other APIs like Foursquare, Songkick, Timeout etc and you will always have an edge over any single big player.