Shares of Societe Generale de Belgique SA soared as Italian financier Carlo de Benedetti launched his long-awaited bid to take control of Belgium's largest holding company. But dealers noted that Societe Generale's shares, which surged 580 francs to 4,730 francs, or about $135.14 each, remained well above the 4,000 francs a share De Benedetti has offered to pay. Dealers predicted that De Benedetti would wait for the price to fall before increasing his stake. The Italian investor claims to be the largest single shareholder, controlling through allies about 38% of Societe Generale's stock. He is seeking to acquire an additional 15% stake.