Margin lending is proving to be a growth product for the few banks that seriously market this financing option.The number of people with margin loan accounts increased to 240,000 at December 2009 from a post-GFC low of 189,000 at March 2009, data published by the Reserve Bank of Australia on Friday shows.So while the slump in equity markets across 2008 and into early 2009 generated a rise in the incidence of margin calls and plenty of adverse experiences for a select customer group, banks and their allied financial planners have succeeded in attracting a new group of customers.The aggregate level of margin lending increased five per cent over the December 2009 quarter to $19.2 billion, or 53 per cent of the pre-GFC high for this type of lending.Credit limits increased four per cent to $55.4 billion or 72 per cent of the pre-GFC high.