I hear "fake news!" in context of mainstream media a lot (in fact, majority of time), contradicting your claim it was invented by mainstream media against independent ones*. I simply view it as mudslinging between kettles and pots of all kinds. This term gained notoriety in last years, but actually fake news are as old as news themself.
* I really hope you don't think alt-media are more credible just because they are independent from mainstream.
In the past, news was filtered by those entities that had control over mass distribution to the public. The public snapped open their news papers and clicked on their news channels on tv. What we knew about the world was given to us through them. Therefore, our opinions on things were as good as the information we were given. As a result, these few media sources could shape the debate, frame the argument, and occupy our much mind space on contemporary matters.
However, now, we see a broadening of access to raw reports and direct source information through the internet, which allows for greater perspective outside the previously dominant filtering system, which breaks the traditional framing of the argument, shaping of the debate.
As there can be fake news from both alternative and main stream media, we now have a greater ability through this broad access to raw information to determine what is credible and was is not, which was never available until recent years. This ability to discern allows for the potential to create a better check and balance on credibility, which can help shape market completion for consumer clicks/fees/listens, thus the rise, in internet distributed news outlets on YouTube, Twitter, etc to compete for your attention... again, something that had never existed before in media system until recently.
As this new access becomes more broadly utilized among the broad market, mass public, we the people begin to be less dependent on middle people for interpretation of source material to directly capable of interpreting the source ourselves in a timely fashion. He are no longer dependent on others to shape our opinions we now have greater control over fashioning our own independently.
This is a massive shift and we living the transition right now. The idea of fake news is as old as time, however we have far more capability of spotting it as individuals then arguably anytime in history.