Williams sold Gowalla to Facebook in 2011. He worked at Facebook for two years, and is now doing a new company.
According to a Facebook page he set up about the search, Mark Williams unexpectedly left his home in Texarkana, Texas on Saturday evening and failed to return.
He is a cancer patient, on medication, and Williams worries he may be disoriented and unaware of his surroundings.
He was driving a 2010 White Toyota Tundra Crew Max truck, Texas license plate DDZ9707. And his phone was located at a truck stop at I-30 and Highway 108 near the Texarkana Airport. Security camera footage indicates that he could have been heading north across state lines.
He's about six feet tall with brown eyes.
Williams worries he could be anywhere in Texas, Arkansas or Louisiana and has desperately been tweeting about the disappearance since yesterday afternoon.
He has reached out to authorities who have put out a Silver Alert, a statewide public notification for missing senior citizens, and is also checking with hospitals across Texas. They have asked anyone with any information on Mark's whereabouts to call 682-235-8375.
In the meantime, members of the tech community have been offering support to Williams on Twitter, trying to keep his spirit up.
Ryan Hoover, founder of Product Hunt, tweeted that he was sorry to hear the news, and Adam Bain, Global President of Revenue and Partnerships at Twitter, created a free targeted ad campaign on Twitter to help get the word out.
Facebook also promoted Williams' Facebook post and has been tweeting about the disappearance from their corporate Facebook design department handle. Williams worked as a product manager at Facebook from 2012 to mid-2013.
"There's been a lot of angst in Silicon Valley about the tech community lately, but you all have been champs today. Thank you," Williams tweeted last night.