I generally agree with you in the sense that these apps will need to raise large rounds of funding just to stay in business because they don't generate any revenue. Their plan is to be bought by a large company like Facebook or Google.
Look at Snapchat; they might regret turning down Facebook's offer to purchase them from over $3 billion, since Facebook purchased WhatsApp instead and Snapchat raised another round which increased their valuation even more.
Now whoever purchases them will have to pay more than what they're valued at, which limits the amount of companies that can do that, since there are only a handful that have that kind of money and would be interested in a company like Snapchat; and at the moment the only likely one is Google since they're desperate to compete with Facebook on social.
Anyways, my point is that these VC backed mobile apps that are designed to grow fast but not generate revenue are really a gamble, since they all depend on being purchased by a large company rather than generate serious revenue on their own. VCs actually tell the founders of these companies not to try to generate revenues since that will lower their valuation.
A company with 100+ million users that has no revenue but is growing fast has great potential, so they're valued high, but a company that with 100+ million users that has 5 million in revenue but is growing it's user base fast has demonstrated that their business model is a failure, so their valuation will lower as a result. VCs play those kinds of games.