Just 15% of people who use TV networks' websites or apps watch digital content synchronized to live TV, translating into just 3% of the overall TV audience aged 13 to 54, according to market-research firm GfK. Users more commonly watch on-demand video clips and look up schedule information.

WSJ: TV Networks Play to 'Second Screen'

By offering so-called second-screen content synchronized with the broadcast, the networks hope they can persuade viewers to watch programming live, instead of on a digital video recorder several days after their initial broadcasts are recorded.

WSJ: TV Networks Play to 'Second Screen'

The SocialTV Timeline launch follows the announcement of ConnecTV's AdSync Network, which enables "view-sers" to act immediately on relevant television ads and promotions by tapping on synchronized companion ads on their second screen devices: Buy now, get a special promotional offer, watch additional product videos, find the closest store or enter a contest.

ENGADGET: ConnecTV adds SocialTV Timeline for mobile, puts chats with stars on a second screen