The best way to monetize is to accept Google's offer for $4 billion.
The funny thing is, you could make the "hard to monetize" argument about so many other startups that nevertheless seem to secure venture capital anyway. Since when has SV investment ever been about the actual viability of the business in question? It's all about the pump and dump.
Secret and Whisper aren't even fads. They are getting a lot of hype but no one is really using them. People are going to start asking why these apps aren't taking off.
The novelty will wear off soon enough.
So what's the difference between JuicyCampus.com and Whisper/Secret? The former was panned by critics and the latter praised. At least JuicyCampus kept it's mud slinging limited to the confines of a college campus.