Dipping its toes in e-commerce, the personalized magazine app Flipboard yesterday launched a way for anyone to create catalogs on the service. The new feature is an extension of a move in March to allow users to create their own online magazines. To date, some 4.5 million magazines have been created, CEO Mike McCue said in an interview. Flipboard's user base has been booming this year, growing from 50 million registered users in March to 90 million today. The new catalog feature, which adds shopping to the app's range of content categories, is likely to be particularly popular with retailers.