Seems difficult to make money with this, but here's a few ideas. They could charge new link sharing sites for the privilege of being available in their widget,

Seems difficult to make money with this, but here's a few ideas. They could charge new link sharing sites for the privilege of being available in their widget, or for better placement. They could start their own link sharing site / link aggregator based on the data they gather from the widgets, then of course also get prominent placement in the widget for the new site. They could show a small ad in the widget.