When Major League Baseball began two domestic-violence investigations last month, allegations against Chicago Cubs shortstop Addison Russell and Tampa Bay Rays catcher Derek Norris did not come from the usual source — a police report, or video, or court testimony.

Instead, they came from social media.

Kristen Eck, Norris’s former fiancée, wrote on Instagram that she had been physically and verbally abused by Norris in 2015. And after Russell’s wife, Melisa, wrote on Instagram that Russell had cheated on her, a friend of Melisa Russell’s posted that Addison Russell had hit his wife in front of his two young children.

Norris, who has since been waived by the Rays, and Russell have denied accusations of abuse. Pat Courtney, an M.L.B. spokesman, confirmed that the league was investigating both cases but declined to say more.

Though the two cases are in their early stages, they highlight a new challenge that baseball faces in addressing domestic violence.