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Twitter (NASDAQ: TWTR) have come up with a new way to target ads. It will “see” which apps a user has downloaded to allow marketers to aim their messages. Twitter needs the service. It has yet to prove that it can bring in the sort of adverting rival Facebook (NASDAQ: FB) does. Wall St. worries about the viability of its current approach to increasing its sales.

Twitter is not the only company that follows app use. The largest distributor of apps–Apple (NASDAQ: AAPL) does similar scanning.

According to a note at Twitter, people do have a way to avoid the action:

To help build a more personal Twitter experience for you, we are collecting and occasionally updating the list of apps installed on your mobile device so we can deliver tailored content that you might be interested in. If you’re not interested in a tailored experience you can adjust your preferences at any time (read below). Additionally, if you have previously opted out of interest-based ads by turning on “Limit Ad Tracking” on your iOS device or by adjusting your Android device settings to “Opt out of interest-based ads,” we will not collect your apps unless you adjust your device settings.

Twitter added it does not gather information from “within” any application’s activity.

Some users may believe Twitter’s discussion of the new app tracking is a bit disingenuous

Twitter is using your app graph to help build a more tailored experience for you on Twitter. Some examples of how we may use your graph data include:

• Improved “who to follow” suggestions that share similar interests.

• Adding Tweets, accounts, or other content to your timeline that we think you’ll find especially interesting.

• Showing you more relevant promoted content.

Twitter already puts advertising into user feeds.

Investors are skeptical enough about Twitter’s attempt to increase sales that, unlike may other tech and internet 2.0 stocks, its shares trade well below their 52-week high, which is almost $75. Shares closed yesterday at $41, against a 52-week low of $29.

The new service will not help unless marketers quickly see its advantages.