Completely aside from the ads or any form of income, consider this:
Things have value because people consider them valuable. Not because of income or assets, but because people's consideration
You invest $1,000,000 today in a website which is receiving 25 million visitors and valued $XX million based on some computation that this is what it is worth to be able to reach this many people.
One year from now, the website gets 50 million visitors. Valuation increases by 100%. Your shares are now worth $2,000,000, even if the site hasn't made a single cents of income.
As an institutional investor, if you are confident that valuation will increase as popularity increase (which is almost a given because money reflects the amount of attention something attracts), and that popularity is increasing on such a great trend, making a small investment worth a few million dollars is reasonably safe.
Another thing you might want to look into is the earning potential of recruitment ads. I remember Joël mentioning on his blog, back when he was blogging, that his job board (on the blog Joëlonsoftware, not the much larger one on SO) was bringing in over a million USD in less than a year and that the jobmarket website business was worth $2bn overall.
Building expert communities on every valuable subject under the sun, they are creating a potential worth which is literally mind-boggling. Fast forward a few years. Insane numbers of users debating in just about any subject under the sun. Start linking all this together. Users start connecting up on a number of different subjects, and progressively, you enter within the realm of social networking on a scale large enough to eat up Facebook.
The potential is huge.