1) Your landing page does not make it at all obvious what Rakable actually does. I already share links privately (via SMS, email, and Facebook, mostly). It's not that efficient, but it's usually about two clicks. I could install plugins to my browser to make it fewer clicks, but I haven't because I just don't care enough.
Your landing page should tell people how Rakable helps them do something they've already been doing, but does it better.
It also needs to be very clear whether the other party must be a Rakable user in order to receive your links. If they must, then that's a dealbreaker. 100% of my friends are reachable via SMS and email, and close to 100% are reachable via Facebook. If Rakable starts out reaching 0% of them, it's not usable. Even if it gets to 90%, it's still not that usable.
2) The "dearth" that you speak of doesn't actually exist[1]. In fact, I'd say there's a glut of services that allow you to privately share links. Many social bookmarking services offer(ed) this feature.
In fact, "social bookmarking startup" became something of a joke a few years ago, much like "photo-sharing app" and "daily deals with a twist" and "Facebook killer". There were just so many pitches and so many indistinguishable startups in those categories that they've become tropes.
Some of the entrants in those areas failed because they were poorly executed, but some of them failed because users just don't want them that badly. Just because a hundred million people want something doesn't mean it's a sustainable business -- they need to want it badly enough to allow you to monetize it. And if you're not seeing a successful [insert startup category here], it's probably because the idea is bad or unworkable, not because no one else has tried to do it.
The question is, what exactly does Rakable do? I'm not going to sign up just to find out. You should tell me on the front page. Does it organize the links I send to others, so they can log in and see all the links I've send them? Or so that I can refer back to them? How is it different from any messaging service?
3) It's often better to start out by piggybacking on existing channels rather than creating your own. Have you considered creating a Gmail plugin or Facebook app that would scour your messages and organize your links by whom you sent them to and when you sent them?
The cost of user acquisition is just massive, so it's better to have some kind of "secret sauce" that makes it easier for you. And when I say cost, I do mean cost. Lots and lots of money.