Two reasons why I think e-mail sucks at group conversation --- not just web-based e-mail, not just live group conversation:
1. You can't edit past e-mails. This means that if you change the date or venue of your party, I have to discard the original e-mail and preserve the new one instead.
Whereas if it's a Facebook event, you update the details, I get notified about the change automatically, and the original, now incorrect details don't exist anywhere any more; I can't refer to them by mistake.
2. Most e-mail clients suck. Most of my friends are not IT geeks, so most of my friends use an e-mail client that sucks. Which means many of my friends use an e-mail client that doesn't have decent threading support, or makes it too easy to reply-to-sender when they meant reply-to-all, or doesn't properly elide quoted text, or etc.
Which means most of my friends suck at group e-mail conversation. And therefore it sucks for them and me.
I realise that group social e-mail conversations work for you, but they don't work for me.