The websites initially banned the trailer for the actor-producer's religious-themed documentary, saying it violated their respective policies concerning spam, E! News said.
The companies later reversed their decisions after Cameron's supporters said they wanted to see the promotional clip, which announced Sept. 24 screenings of the film across the United States and a live question and answer session with Cameron planned at Liberty University in Lynchburg, Va.
"Because of your firm, loving, and clear voice, not only did Facebook welcome us back, YouTube also removed its block on our 'Unstoppable' movie trailer. We are back online with full access. Thank you!!" Cameron wrote on his Facebook page last Friday.
As of Tuesday morning, the clip had been viewed nearly 200,000 times.
Cameron, 42, was a teen idol in the 1980s thanks to his wildly popular sitcom "Growing Pains." In recent years, he has focused on his faith and religious-themed acting projects such as the "Left Behind" film series and movie "Fireproof."