If you properly protect your security number, which requires some monthly commitment (such as $15 lifelock, or freecreditreport $5 per month), you can pretty much post your SSN online and really not much will happen. Any time someone uses it, you will get the alert and chance to act (stop the inquiry before it hit hard).
It just that most people believe that not making their SSN public is enough for it to be safe.
The CEO of Lifelock posted his SSN in several ad campaigns to tout the effectiveness of Lifelock. And, surprise, his identity was "stolen" 13 times[0].
1) There are free sites to monitor your credit such as Credit Karma. No need to pay hundreds a year. They even send out emails whenever you open up a new line of credit. Lifelock is a huge scam and has been fined by the FTC.
2) Just being alerted when someone else opens up credit in your name is hardly "protection." They still opened up credit in your name and you have to deal with that which is at the very very least inconvenient.
3) Posting you SSN online makes products more expensive for everyone because companies at the very least have to devote extra man hours every time someone else tries to take out credit in your name. Even if nothing happens to you they may have already issued a loan to the person and now has to write that off.
4) This is about fraudulent tax returns which credit monitoring companies wouldn't have info on.
I've had my identity stolen and I can tell you,it is truly awful.
It just that most people believe that not making their SSN public is enough for it to be safe.