Shares in the mainland's largest e-commerce portal slumped 4.7 percent to HK$15.44.
On Saturday, Microsoft walked away from talks to acquire U.S. internet company Yahoo after disagreements over the sale price.
Alibaba.com shares have been rising in the past few months on news of the planned takeover of Yahoo by Microsoft.
"Investors had been buying this stock on hopes that Microsoft will buy into Yahoo but with the acquisition failing to materialise, disappointed investors are dumping shares," said Ben Kwong, chief operating officer at KGI Asia Ltd.
Yahoo owns 39 percent of the Alibaba group, the parent company of Alibaba.com.
($1 = HK$7.8)
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