After being passed up by Google for an acquisition in the $200-million range, social news site Digg is going it alone with a fresh $28.7 million infusion of capital. The C round was led by new investor Highland Capital Partners, which as joined by existing investors Greylock Partners, Omidyar Network, SVB Capital. That brings the total amount the company has raised to $40 million since it was founded four years ago. Founder Kevin Rose may have taken some cash off the table, Om is reporting.

The company has big plans for the new cash. It will double its current staff to 150 over the next year or so, move to a new corporate headquarters, localize Digg sites for international markets, give publishers better analytics, and spend more on marketing. Digg will also no doubt try to improve its service by making its recommendations even better and more targeted to each individual member. (A partnership with Facebook that is about to launch should prove interesting in this regard).

Digg claims 30 million monthly unique users. ComScore, however, counts 15.8 million uniques worldwide as of August, and 6 million in the U.S., both of which were down from July. (Quantcast shows 21.4 million global uniques, and 11.4 million U.S.). At least in the U.S., Digg’s traffic seems to be flattening. So you can see why the company wants to emphasize global expansion. Here is a chart of its U.S. traffic:

And here is a chart of global traffic: