Zynga Inc., the largest maker of games for Facebook, declined in its first day of trading after raising $1 billion in an initial public offering that gave it a greater valuation than rival Electronic Arts Inc. The shares, listed on the Nasdaq Stock Market under the symbol ZNGA, fell 5 percent to $9.50. The developer of games such as “CityVille,” “FarmVille” and “Mafia Wars” sold 100 million shares for $10 each, the top end of a proposed range.