In the sentence
I find Area 51 interesting because there is so little information about it.
Shall I add 'to be' before 'interesting'?
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The following sentences are from MW Learners Dictionary:
I found him (to be) a sensible man.
Students often find this book (to be) useful.
The Dictionary has put the "to be" in parenthesis, which indicates that it's optional to use the "to be".
So you can either put "to be" in front of the adjective interesting in the OP's sentence or leave it out, without any difference in meaning. However, the structure of a sentence with the verb find + object + adjective is more common than that of find + object + to be + adjective.
"To be" or not "to be", that is the question. . .
They are both grammatically acceptable. The use of the phrase "to be" in this case sounds more formal and a little archaic, which is probably why I prefer it, but the construction without "to be" is far more common, at least in American English.