A "corporate embassy" doesn't have anything to do with a "government embassy". While a government embassy represents a government in a foreign country, a corporate embassy represents a corporation in a foreign country.
A corporate embassy does not enjoy any special protection under the Vienna Convention. Legally speaking, it is usually just a branch office of the company. Although in some cases it might officially operate as a branch of a local company, for example if the government does not officially allow that foreign company to operate within their borders (something the Chinese government is very picky about when it comes to some industries). In this case it might enjoy even less legal protection than an official branch office.
In the case of Facebook's "corporate embassy", the declared mission is to be
[a] center, where Chinese who have hardly any experience with the social network can learn about it and figure out how to advertise on it.
Its main purpose is to find Chinese companies which might be interesed in advertising on Facebook. The facebook.com website itself is blocked by the Chinese government. So Facebook is unable to sell their customers any attention of Chinese consumers. But they can sell them the attention of consumers from other countries who might be interested in buying products and services from China.
This is a purely commercial purpose, not a political one.
However, a "corporate embassy" might also have a political purpose in form of serving as a base for political lobbying. If the Chinese government could be persuaded to unblock facebook.com, then that would certainly be something the Facebook shareholders would be extremely excited about. But whether the Facebook office in Shenzhen does seriously pursue this goal is a matter of speculation.