The ranks are thinning at Weight Watchers International Inc.’s diet meetings. The company forecast full-year earnings below Wall Street expectations because attendance has been low so far this year, Reuters reported. By the time the first quarter is over, Weight Watchers forecasts, there will be double-digit ­declines — in the mid to high teens, percentage-wise. The company expects full-year earnings of $3.50 to $4.00 per share. Analysts forecast $4.75.