I don't think it's terribly controversial. Consider two Twitters: the present one, and another devoid of all users and their content. Does one have less value than the other?
We're already engaged in dialog which respects content as capital. Consider Ben Lee (legal counsel to Twitter) who said, "Twitter users own their Tweets." How is it that a tweet is ontologically capable of being owned? Well, it's intellectual property and as such an object capable being owned. Moreoever, it's owned by author, not Twitter. If you answered in the affirmative above, then it's even participating in the circuit of value creation. Yet, the production of tweets contributing to Twitter's value happens largely for free. It's quite simply benefiting on the backs of free labor.
Anticipating your next point, yes, Twitter users willfully choose to participate, but this doesn't negate the process that's taking place. Free labor creates capital.
This process is markedly different from your McDonald's example. It would be akin to people voluntarily supplying McD with beef patties for free. Happy to discuss that though. I'm fairly certain it would be a fruitful discussion.