I believe it would also extend to anything that can be confused with the white and blue logo in the context is social media.

I can’t take the Android droid logo, make him blue, give him a squiggly antenna, and then try to make him the logo of my new phone company.

While Meta doesn’t own the letter X, if the government says “People might get confused between these two marks” that’s a valid reason to reject the trademark or prevent the company from calling itself that. See https://www.uspto.gov/trademarks/search/likelihood-confusion:

Likelihood of confusion exists between trademarks when the marks are so similar and the goods and/or services for which they are used are so related that consumers would mistakenly believe they come from the same source. Each application is decided on its own facts, and no strict mechanical test exists for determining likelihood of confusion.

So basically it would come down to a judge deciding if the marks are too similar to each other or not.

To determine whether a likelihood of confusion exists, the marks are first examined for their similarities and differences. Note that in order to find a likelihood of confusion, the marks do not have to be identical. When marks sound alike when spoken, are visually similar, and/or create the same general commercial impression in the consuming public’s mind, the marks may be considered confusingly similar. Similarity in sound, appearance, and/or meaning may be sufficient to support a finding of likelihood of confusion, depending on the relatedness of the goods and/or services.

So I could use something similar to the Android logo to sell fishing supplies, since the likelihood of confusion is small - Android doesn’t make fishing supplies. We only have an issue if I start selling phones or if Android starts selling fishing supplies.