Twitter has a problem: A number of users tweet, then lose interest. It needs a way to reengage them in the site. And tonight its starting to test one way: Notifications. The test is currently only rolled out to a limited number of users right now, according to this update. But those who have it should notice an indicator similar to what Twitter does to let you know there are new search results on a query (see a capture above and below). Theres another service that does these types of notificatio