This is a great question about etymology! As thranax states, Pinterest is a proper noun. It is not going to become a "real word".
However, as the use of Pinterest becomes exponentially of more interest around the world, other words might slide in. For example, "to pin something" (on Pinterest) has a connotative meaning unlike the denotative dictionary definition of "to pin". It would take some more years of this terminology being used in modern English before being considered for the dictionary, but that is how new words are "made".
Somewhat like the Pinterest as a proper noun example, the Xerox company got its name in the dictionary. However it is not used as a proper noun. Society adopted the term "to xerox something" to mean make a photocopy of it. It took a few years, but it is there and there are still people that use that term.
The English language is interesting as it does grow as society changes. As long as YOLO, swag and bromance do not become real words, I am a happy man.