Doug Belshaw responds to the question as to the biggest mistake when it comes to digital literacies. After pointing out that they are plural, context-dependent and socially-negotiated, he explains that it is not something that one necessarily becomes.
There is no stance from which you could call someone ‘digitally literate’, because (as Allan Martin has pointed out), it is a condition, not a threshold. There is no test you could devise to say whether someone was ‘digitally literate’, except maybe at a very particular snapshot in time, for a very defined purpose, in a certain context. (Source)
I have elaborated on Belshaw's book here.