Have you heard of Boston based Sponty?
http://www.thesponty.comOur original idea was exactly what you describe: organize casual events with friends. But that didn't gain traction.
What we've empirically verified, is that this is not a big enough pain point to get people to ditch the default position. The default position being:
1. SMS
or
2. Facebook status updates. Not even structured events, even though Facebook just made creating the latter much easier.
So we pivoted. The new incarnation was:
Discover events through taste makers.
This -we thought- allowed us to short circuit the "I don't have enough friends on the service" argument. So now you can follow people who share your cultural interests and discover events through them so that you can drag your friends along.
This newest incarnation is too hard to distribute. By distribution I mean: user acquisition. We had/have an awesome website, a native iPhone app, cool graphics and even a gaming element.
You need to convince enough people to post events, and then convince others to follow them and like/attend those events.
So my 2.0 cents:
1. Think about distribution: How are you going to get users?
2. Is this a big enough pain point to convince people to ditch the default? (I don't believe so, and I brought a similar product to market).
EDIT: Sponty is still running and we do have users. But we've moved on and are now working on social games.
Cheers,
M