The deal values on the same $15bn terms as the stake Microsoft bought in October and marks only the second time that Facebook has sold a share of the business to an outside investor.
The acquisition of the stake, which did not involve the billionaire's companies such as 3's parent company, Hutchison Whampoa, and TOM Group, gives Li a 0.4% share of Facebook. Microsoft's holdings represents another 1.6%.
However, the move will fuel speculation that the deal could help Facebook tap into the burgeoning advertising market in China because many of Hong Kong-based TOM Group's businesses are geared towards this market.
TOM's businesses include outdoor media, publishing, television and entertainment, and internet services.
The investment was made through the Li Ka-shing Foundation. According to an annual survey by Forbes magazine, Li is the ninth wealthiest individual worldwide, with an estimated $23bn fortune.
The investment means the online social network, which is just three years old, now has support from the richest man in Asia and the world's largest software maker, Microsoft, which paid $240m for its 1.6% stake.
Facebook is planning to boost its profits with new ways for advertisers to connect with its audience of 55m users, now the second-largest social network on the internet behind News Corporation's .
Last month, Facebook introduced a marketing initiative called Beacon, which circulates potentially sensitive information about the online purchases and other activity made by its users.
However, Beacon was met with strong opposition by thousands of users, leading to Facebook promising that it would no longer share any information collected in the Beacon programme unless users gave their explicit permission.
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