The problem is that you said "monetize later" rather than "become profitable later".
For example, someone else suggested that Google was an example of a company that had no way of monetizing their original product. That's false. In the earliest days of Google it was obvious that there were tons and tons of ways to monetize the product -- hence the very early-stage investment -- though there was uncertainty of which ways of monetizing were best for growth and profit.
Reading around the history of Facebook, there were advertising sales quite early, a lot earlier than people realise. They used to do local campus deals, student credit card deals etc.
Echoing dasht's point above, no startup should plan to "monetize later". Plan for 'profitability later' but without some kind of test (like a sale) it's going to be harder to test and iterate on ideas (lean startup etc.)