Twitter has introduced Promoted Tweets in the timeline of its mobile apps, reflecting the fact that 55% of its active users log in through a mobile device.

In the coming weeks Android and iPhone users will begin seeing Promoted Tweets near the top of their timeline from brands they already follow. 

blog post from Twitter said this will “ensure that people see important tweets from the brands they care about.”

Twitter has been showing ads in app search results since last year, but this is the first time that Promoted Tweets will appear in the timeline and in ‘Who to Follow’ recommendations on the mobile app.

The move shows that Twitter is serious about ramping up efforts to monetise its 100m active users.

Two weeks ago it opened up its self-serve ad platform to another 10,000 marketers after an initial trial phase involving just 100 clients.

And yesterday we reported on a new deal with DataSift to make available two years of historic data from the social network.

Ad sales are Twitter’s primary revenue source, with eMarketer estimating that is generated $139.5m in ad sales in 2011.

Social networks have traditionally been cautious about placing ads in user’s timeline.

Facebook only recently began trialling newsfeed ads but limited it to one ad per user each day.

Twitter says it “strives to create products that put users first, and continue to enrich your Twitter experience.”

Yet realistically, this latest update is purely a business decision.