Why is Facebook planning to go public now?

I just read about the plans of Facebook going public with a 10b$ share offering. I am sure that this is going to generate a lot of press and noise around this IPO.

The question to ask is this: Why go for an IPO? Why now?

In my opinion, the investors of facebook have realized that

1. The dizzying growth rates in membership is no longer sustainable.
2. The scope for breakthrough innovation is also reduced which could add substantial value to the company.
3. Other players (Google+, Twitter) are catching up fast.
4. The valuation for the company will only fall going forward as the growth rate slows down. So, better to cash out at the peak or atleast close to the peak.

In my opinion, the following will happen post the Facebook IPO:

1. Facebook will continue to perform well due to its momentum.
2. In about 3-6 months, we will start seeing some decline in the growth rate and the momentum.
3. In about 12-18 months, Facebook will become another large corporation living on the laurels of their glorious past.
4. By this time, Facebook will have a lot of cash in thier coffins and so will start an acquisition spree to continue to keep the growth coming.
5. In about 20-24 months, we will have some other start-up take the lead and Facebook will start receding in the background.
6. Meanwhile, all the investors in Facebook would have cashed in the IPO will move on to fund the next great idea.

I only hope that I am proved wrong and that Facebook continues to remain one of the innovation hubs and continues to grow in the years ahead. However, I am not very sure if that is the future of Facebook.