Is it an antitrust violation for Twitter to block Mastodon links?

Under current law, a good starting point is LiveUniverse v. MySpace, No. CV 06-6994-AHM (RZx), 2007 U.S. Dist LEXIS 43739, 2007 WL 6865852 (N.D. Cal. June 4, 2007), which held that MySpace could disable user-posted links to vidilife.com. Per the court, it was not a Sherman Act § 2 violation because MySpace had no duty to deal with vidiLife, and because MySpace wasn’t preventing vidiLife from competing in the relevant market.