The rumours raced across these woodland communities faster than the fire: the Groveland firehouse had burned down; authorities were cutting power lines to force people to evacuate; police had arrested an arsonist who started it; the sequoias were going up in smoke.

None of it was true but the speed and reach of social media, especially Facebook, convinced many otherwise, compounding alarm amid one of the biggest wildfires in California's history.

"Social media is a wonderful tool but I've seen here it's not always our friend," James Mele, the sheriff-coroner of Toulumne county, told hundreds of residents at a town hall-style meeting on Monday night.

People listened in on authorities' radio communications and disseminated fragments which often misled, he said. "Folks pick up chatter and it spreads."

He debunked a rumour that authorities cut power lines to encourage recalcitrant homeowners to obey evacuation orders. People should follow instructions, not gossip, he said. "If we come to your door there's a reason. It's time to go."

The deployment of more than 3,700 firefighting and support personnel, 15 helicopters and two DC-10 jets has crackled the airwaves with reports, commands and acronyms. Even if you don't intentionally eavesdrop you hear fragments – "more air support, now" – from the walkie-talkies of firefighters resting by roadsides, or in cafes.

Twitter, Facebook and other social media can swiftly amplify an incomplete or misinterpreted message, officials complained, but they also credited Google with directing people to reliable information such as the inter-agency emergency site Inciweb.

The blaze grew to about 280 square miles on Tuesday, up from 252 square miles on Monday, partly because of controlled burns by fire crews. Authorities said 20% was contained, up from 15% a day earlier, stoking cautious optimism that they were winning the battle.

The Rim fire has devastated forests, destroyed 31 homes and 80 outbuildings and continues to threaten historic sequoia groves and San Francisco's water supply. Given its magnitude the damage to human infrastructure so far has been relatively slight.

Residents have remained largely calm during the ordeal but the digital rumour mill has at times churned anxieties into terrors, including the report of the Groveland firehouse becoming a grim pyre. "That would be a surprise to the firefighters sleeping there," Tuolumne county Sheriff Sergeant Scott Johnson told the Modesto Bee.

"The problem is there's no way of verifying that information. If people didn't exaggerate things and actually gave out factual information it would be a very good tool."

The cause of the fire, which started on 17 August in a remote canyon of the Stanislaus national forest, remains unknown but false reports of police arresting an arsonist conjured the spectre of an evil or insane pyromaniac.