In several instances, the union used Facebook to rebut rumors being disseminated on plant floors or in the news media, rather than allowing them to spread unchallenged. Shortly after a Detroit television station reported that workers would get a signing bonus of $7,500, a message posted on Facebook from Jimmy Settles, the union’s vice president in charge of Ford negotiations, described the report as inaccurate and “designed to intentionally create false expectations.” The finished deal included a bonus of $6,000 for most workers, some of whom had begun posting on Facebook that they would vote against any contract with a bonus of less than $15,000.

“It allowed us to get to the membership quickly,” Mr. Settles said in an interview. “The one thing we always had to combat was the expectations of our members. Historically, we didn’t have the apparatus to get that information out.”

The Chrysler team reacted similarly last week to quell speculation that the talks were headed to arbitration. Chrysler workers agreed in 2009, as part of the company’s government-aided bankruptcy, to give up their right to strike and that any impasse would be sent to binding arbitration, a result that both parties have said they want to avoid.

Art Reyes, the president of U.A.W. Local 651 in Flint, Mich., said Facebook had gone a long way toward “demystifying” the complex negotiations.

He has been active on Facebook himself, posting frequently on a page he created for his local and participating in many conversations on the U.A.W.’s G.M. page during the talks. Mr. Reyes represents a G.M. parts processing plant staffed entirely by entry-level workers, many of whom are in their 20s, new to the bargaining process and more likely to engage one another online than at the union hall.

“They’re used to expressing themselves on Facebook or on Twitter. Getting real-time answers is something they have an expectation of,” he said. “Nothing feeds the rumor mill like a lack of information.”

To be sure, the union still has kept most details of negotiations closely guarded, leading some members to grumble on Facebook that they are still in the dark. In fact, complaints — about the secretive process, the union’s leadership and the finished contracts — have become common, as they are on many Internet forums. But the page administrators say they have let most critical comments stand uncensored, stepping in only when they spot inaccurate information and to delete posts with foul language. After the G.M. contract was settled, however, the union noticed many comments from retirees who were angry that they would not receive their traditional pension increases or Christmas bonuses, and responded with a long statement.