Why on Facebook on the button in the menu block an article "a" is missed. I have a guess, but not sure if I'm right. Could it be because of all the inscriptions on the button are cut?
Have you noticed that in this site there is a button "Ask Question"? What happens with the "a"? :-)
I don't know the exact terminology for this phenomenon but it's a convention, a standard, it's very common. Buttons are usually labeled with just a verb [action] and a noun [object]. The noun represents the result of the action executed when you click the button. You would include the "a" if there is another button for creating two pages at once or n pages.
There is a branch in the IT sciences named User Experience that studies how you have to label your buttons, distribute the elements in a screen, etcetera ... They have even their own site in Stack Exchange. Here is an example of their posts, talking about how to label buttons.
The reason is for readability. Like how news headlines remove all words like 'and' and 'or' and replace them with commas. The fewer the words, the quicker the read. The fewer the punctuation, the quicker the read.
You would never tell someone 'create facebook page'. You would definitely say 'create a facebook page'.
While the second is correct, shortening statements to quicken readability in situations where they do not need sentences is quite normal.