How do we prevent a large competitor from just implementing our feature?

The reality is that Facebook could implement the same feature. However, if the feature is patentable, and you file a patent application before anyone else does, you could pay back your investors with an enforceable patent. The key is a detailed patent feasibility study and a well written patent application that can be enforceable if it issues.

If the feature is patentable, then the protection you receive from that patent should be sufficient to either prevent your competitors from implementing the feature, or to sue them when they do. (Features that are easy to detect when implemented are great from the latter perspective, but even for highly embedded functions/algorithms/circuits/etc, there ARE ways to reverse engineer the product and prove infringement.) The easiest way to prove to your investors that you will have this protection is to file for a patent application or at least a provisional patent application. Performing a prior art search would also be very useful to show that your feature is not already patented by others.

In software, even if you patent it are you able to defend it? Look into the cost of litigation insurance. If all you have is a "feature" then perhaps the problem is the business model (insufficient differentiating value) and not the feature per se?

I'd point out that first mover only works under certain circumstances and fast followers, especially the chinese hordes is impractical to defend against. See my post on Dunedin StartupWeekend on indian rope trick.