Unless he does some remarkable philanthropy, Zuckerberg will be forgotten shortly after Tom from MySpace. Facebook isn't really fundamental to anything, it's just a business that executed an existing concept very well.
RMS may never be well known to the general public, but his reputation within his field is likely to endure in a manner similar to Euclid or Eratosthenes. GNU and the GPL are foundational elements of modern computing. Stallman had the fundamental insight that software is inherently political, then built a large part of the infrastructure necessary for humanistic software to flourish. The landscape of modern software would be utterly unrecognisable without the contributions of RMS.
So is McDonald's, but I somehow think that they've managed to embed themselves into the culture/society enough that they won't soon be forgotten should they ever fall.
You're overestimating how famous this guy is.
Hyperbole much? He's not going to be forgotten, but... RMS is a hacker icon, but he's a bit obscure outside the community. RMS is legit hating on Facebook here but there's no Hollywood movie about RMS. Zuckerberg is a household name. I'll agree that RMS has done more net good thank Zuckerberg, but who is more likely to show up in a textbook a hundred years removed at this rate?