Competition for mobile advertising dollars is set to intensify as Facebook joins Google, Apple and Twitter by launching a mobile ad network for generating revenue from users accessing apps outside its domain. At the same time Facebook is also planning to expand into location based mobile advertising after preparing the ground with the launch of a feature called “Nearby Friends,” allowing its users to share their location with a selected group of friends. Facebook has indicated that this will support location based advertising on an opt-in basis. With these two moves Facebook is addressing two of the principle mobile ad sectors, location and in-app ads. This comes at a time when the mobile advertising market as a whole is growing…