Contact recruiters to (1) discuss challenges sourcing talent, & (2) gain validation of a platform that would bridge the gap between the technical knowledge of a recruiter & that of a developer.
I'll answer the question two ways:
First, a note of caution. The recruiting space is incredibly crowded (I'm an angel investor in a company in this space) and there are already a few companies addressing exactly the pain point you're describing, mostly through social proof, contesting, and other venues.
Also, generally, the social recruiting space is *really* hard to raise money post seed. There are so many companies that have raised great A rounds who just haven't lived up to investor expectations in this arena, that there isn't a lot of excitement that people can "beat" the execution of the "best" which haven't been "good enough."
So if you're thinking about building a business here and plan on raising venture funding to scale the business, think very carefully about whether this is really your passion and you and your team will be good enough to be the best, keeping in mind that there are some really smart people who have tried before you.
Now to answer your actual question: Recruiters, especially technical recruiters, live on LinkedIn. You can easily find them, and I'd literally do a search of the top tech companies that aren't LinkedIn, Facebook, Google & Twitter. They already have such well-tuned machines, that their people, no matter how junior, will talk about their existing systems vs be open to a new platform.
Think about fast-growth, billion dollar plus valued companies that are still looking to "fine-tune" their recruiting machine. e.g. Snapchat and all mobile messenger companies, the best and fastest growing eCommerce companies, even Pinterest is still putting together their recruiting engine. Use LinkedIn search to find the recruiters, look specifically for people who have been on the job for less than a year, and then send them contact requests. The more technical your own LinkedIn profile looks, the more likely they are to talk with you.
Happy to help in any way I can via a call. I know this space pretty well, and despite all the concerns noted above, still believe there are opportunities to build great businesses.
You mean to pick their brain? If so , linked in search, send request and short message (ie don't pick brain in opener) then call them on the phone next day and ask them q's. Booking time to chat via linked in prob good idea too given their interviews Scheds. Ps if looking for mass feedback tell them you'll send survey link after your call. Recruiters like the face to face, to be heard etc. should work out well for you