Do you think there exist a privacy risk if the wallet is linked with Facebook or other social media?
Dude, Meta is an advertising company, of course, they want to know further whether you are interested in cryptocurrencies or not. It gives more money for them, and also it suits with their ideal of the metaverse. And also, Twitter has done that, would be naive if Meta didn't catch up.
Will facebook users will still have the option to use metamask or facebook will develop his own wallet?
As a start, they can/able to simply connect the address with the current popular
web3 wallet, that seems the reasonable thing to do even after it's been established, noting Twitter also does that. It gives both the user and the company not much of a hassle since the targeted users are already familiar with it.