I'm trying to claim prior art on the "Twitter" patent. But if that prior art only ever existed as a web page, and only now exists as a Wayback Machine snapshot, does it qualify as legitimate prior art?

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Web pages that are part of the Internet Archive (ie. found with the Wayback Machine) are web pages that were once public, and may be considered "Printed Publications" as per MPEP 2128.

Of course, web pages have been used as prior art during patent prosecution before. For instance, during Amazon.com v. Barnesandnoble.com (1999):

Pages from a website were relied on by defendants as an anticipatory reference (to no avail), however status of the reference as prior art was not challenged.

The Wayback Machine is seen as an effective tool for searching for prior art that is no longer generally available online. But there are some drawbacks to using this tool. For instance, as Intellogist points out, dating previously published materials can be tricky.

First, snapshots of a certain site may not be taken very frequently, which means that you can use the archive to prove a date when content existed on a page, but not when it was first added. Secondly, the archive does not index pages which contain a robots.txt command, which specifically prohibits web crawlers from capturing the content.

TLDR: Public websites that are no longer available may serve as valid examples of prior art.

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