FB/ Twitter Standalone Effort a Practical Alternative?

Is it practical to expect that Twitter and FB, given an adequate campaign, can be used to help develop interations to the point that Google/MSN/Yahoo must not be relied on, or even in combination with?

There is one project I have, which may choose to start with FB/Twitter. It happens to be a niche job board. Rankings with the engines have fallen off dramatically over the past year and a half, with the economy and building, as have their resume and paid company postings.

Might this be a candidate for these two?

I can see connecting with say, recruiters and HR people, in this example, but job candidates – might be flighty. Seeming to want engagement one day but then withering away as they find work.

Not exactly heavy repeat end-users, like buying clothing from their favorite retailer. How necessary is the repeat factor when viewing SM feasability? Does it mainly hinge around degreee of purchase involvement or pass-through advertising (who they follow/intereact with) at least as important as a weighted factor as well

‘Niche’ might be the operative word here.