I know that the reason why people love it, is the intense amount of user interactivity - in other words real-time interactivity.

But I would like to know more.

Is it because of the navigation, is it because of the colors, is it because of the clever logos or icons?

I need feedback. I'm doing a project similar to facebook, but not in the social media industry, however.

What made you go like "Hey, I'm on facebook! I love it!" what made you want to put out word of mouth?

How can I improve what facebook isn't improving user experience?

I think the design is only secondary, look at popularity of site like MySpace when they launch (terrible quality, millions of users). People like looking at photos and saying what they're doing. Also I think the way it create a 'dashboard' of your life, ie photos, friends, news, what people are saying... draws you in, like you said, 'real-time interactivity'.

I think Facebook's growth in popularity comes from their evolution to being the one stop shop of wasting time. It's really how all of their components integrate with one another. On MySpace, they have those same features, but they feel very independent from each other.

Well, obviously many people has too mane spare time to browse the web :) Twitter saves time and works much better for me.