Buy.com Enters Social Networking Scene By MARK JEWELL    BOSTON (AP) -- Online retailer Buy.com is inviting itself into the expanding social networking scene, acquiring a Web site that rewards members with checks or gift certificates if they persuade others to buy their favorite products.    The purchase of Metails.com piggybacks on a growing phenomenon: online services that link people with common interests and friends, creating bonds that transcend social, economic and geographic boundaries...