Since they are going to go through most of the downside involved with an IPO anyways, they might as well get some cash too.
Plus, share/option holders (employees, investors, etc) are probably clamoring for liquidity. It's great if you own a hundred million dollars of FB stock on paper - but it's even better if you have the option to sell and diversify your holdings.
In other words, they NAT'ed the shares.
To facebook, goldman sachs is a single investor. However, many individual investors are part of that GS banner.
In much the same way, with a NAT, there may be many computers within an internal network (but to the outside world there is a single external IP address)
The largest secondary market (secondaryMarket) allows the company to list the terms of who, how and when their stock can be traded.
I doubt any of those are a primary concern - I think it's much more likely that since they broke 500 shareholders last year they've just assumed they would. With a calm, up market it's probably a better time to IPO than recent conditions. Being able to put $10B in the bank for a rainy day is nothing to sneeze at.
However, registration means that they have to publish almost the same information as public companies do and be subject to many of the same regulations. Since there are some benefits to going public, such as liquidity for current shareholders, few companies choose to register and stay private for very long.
It could have advantages in hiring, where now, people can actually realize the value by selling their options/stock.