After the bell, shares unloaded 20.1% to $24.45, after gaining 3% over Thursday's session.
The weight-loss services provider warned of a difficult year ahead. Fiscal 2014 earnings are expected between $1.30 and $1.60 a share, sharply lower than consensus of $2.78 a share, according to analysts surveyed by Thomson Reuters.
"While we are confident that we are on the right track to execute a successful transformation, 2014 will be a very challenging year," said CEO Jim Chambers in a statement.
In the three months to December, net income of 54 cents a share fell 7 cents short of consensus. Quarterly revenue of $366.1 million fell 11% from the year-ago quarter, but came in $8 million higher than expectations.
Total paid weeks declined 8.5% compared to the year-ago quarter, with online paid weeks dipping 6.5% and meeting paid weeks down 10.9%. Declines were driven by a lower active base at the beginning of the quarter and lower recruitments throughout the period.
In its third quarter ended September, the company tanked after warning that it expects trends of declining memberships to continue into 2014.
"While we are working aggressively on both near-term commercial activities and longer-term strategic initiatives, 2014 will be a very challenging year," Chambers said in a statement at the time.
Year to date, shares are down 7.1%.