30,000,000 people using anything has got to be worth at least 9 figures.

$33 per user? That's optimistic.

$33 per user could make sense if this was a defensive acquisition as other HNers have suggested.

If G+ bought Instagram and it made enough of a difference in G+ activity to make G+ competitive with Facebook, I could see how it could knock $1 billion or more off Facebook's IPO price.

Remember, it's 30 million of the more active social network users. These are the same type of core users that Facebook had back in its early days that helped attract the other 750 million users it has.

My app has 7 users. Can I haz $200?

Is Google likely to acquire you? If they do, is it likely the acquisition will lower Facebook's IPO price?

Honest question about this: my buddy and I have a Facebook app with 21M registered users, with about 10% of that being monthly active. How would we compare?

If you have a Facebook app then FB can shut you down. You're not a threat. Likewise, if you have only a facebook app, then you are encouraging people to use facebook, so you are helping facebook.

I guess I didn't phrase my question clearly. Perhaps I should ask how does the value of a user on a Facebook app differ (qualitatively and quantitatively) from a mobile app user?

Active users are worth this much. Whatever you do that attracts and retains users will become valuable.

Processing several million photos a day from thirty million users is worth something.