The sole purpose for an intranet is to facilitate the work of staff in pursuit of the organization’s objectives. Nothing more, nothing less.
A good metric for this is that one of the first things that staff do when starting a project is to voluntarily create a space for it on your intranet. If the intranet is considered a prerequisite for success by staff, then you have succeeded!
I was prompted to post this after reading Nick Besseling’s rant on how stickiness is not a good goal for an intranet (I agree).