It's a bad Monday for billionaire investor Bill Ackman

The week is off to a bad start for Bill Ackman.

Herbalife shares were up about 8% Monday morning following news that it had talks to go private.

The surge is a blow to Bill Ackman's Pershing Square Capital, the activist fund that famously went short on the supplements company five years ago while accusing it of operating as a pyramid scheme. Pershing Square's bet has largely gone the wrong way since it was put on.

Meanwhile, the human resources company ADP rejected Ackman's board nominations. Pershing Square took an 8% stake in the company earlier this summer and had been seeking three board seats, including one for Ackman.

ADP "determined that none of the Pershing Square nominees bring additive skills or experience to ADP's Board," the company said Monday.

"Unlike Mr. Ackman's nominees, ADP's directors have a deep understanding and appreciation of the current state of ADP's business and its clients," John P. Jones, Non-Executive Chairman of the Board, said in the statement.

ADP had been gaining up until July 31, a few days after Bloomberg News reported that Ackman had taken a stake in the company. The stock has been falling since.

Pershing Square Holdings, a publicly traded vehicle which is a proxy for Pershing's private fund, is down 1.7% this year through August 15, meanwhile.

A spokesman for Pershing Square didn't immediately comment.