Most people should not under any circumstances sport a cell-phone holster. There are people and occasions that are holster-worthy, but they're few. These people and occasions tend to involve outdoor work. On the other hand, the sight of a USB flash drive (or pen drive, thumb drive, key drive, or whatever you like to call it) clipped to a key-holder or even to a lanyard tends to spark conversation, if only about price and provenance. People who don't know about them are always curious. Word of mouth must certainly account for a lot of the increase in their use, especially because a lot of workplaces won't spring for them and are also opposed to their use for security reasons, except for those types who, say, might have reason to transport a PowerPoint presentation in that way.