Your problem is that you are not using CDataProvider in the correct way. CGridView needs an instance of CDataProvider to display, but you have used CDataProvider->getData() instead. I'd also recommend using CSqlDataProvider, which is purpose made for custom sql queries. Try this in your model;
$count = Yii::app()->db->createCommand('SELECT COUNT(*) FROM fc_product')->queryScalar();
$sql="select member_count,seller_count,
visitor_count from fc_product fp
group by member_count,visitor_count,seller_count
order by member_count desc";
return new CSqlDataProvider($sql, array(
'totalItemCount' => $count,
'sort' => array(
'attributes' => array(
'member_count', 'visitor_count', 'seller_count',
),
),
'pagination' => false