The danger of giving away an online service in order build a valuable user base often occurs when you start attaching strings.

So when Twitter introduced its most prominent advertising feature to date last week, adding "Promoted Tweets" to users' main timelines, the gambit aimed at generating greater revenues risked provoking members' anger and stirring up a good deal of skepticism.

Users will now see tweets from companies and organizations that they follow appear near the top of the central stream of 140-character messages when they log in, regardless of when the ads are posted.

Now we know Twitter's business plan but will it work? How these Promoted Tweets play to its base is only one part of the equation. Users get used t