Mr Clegg, the former deputy prime minister who led the party during its five-year spell of coalition government with the Tories, has taken a job as the social media company’s head of global affairs and communications.
The 51-year-old will reportedly earn more than £1 million a year in the lobbying role.
Lord Ashdown said his former colleague’s legacy would depend on whether he could help instil liberal “values” at the social media giant.
Political veteran Mr Ashdown tweeted that Mr Clegg’s “reputation as a powerful voice for liberalism and democracy will now depend on his ability to persuade Facebook to be a global campaigner for the same values”.
Mr Clegg lost his seat to Labour in 2017. He accepted the Facebook role after “months of wooing” by the firm’s CEO Mark Zuckerberg, according to the Financial Times.
Former politician Mr Clegg, who starts his new job on Monday, said one of the big challenges he hoped to help the company negotiate was upholding “the integrity of our democratic processes”.
He explained: “It is time to build bridges between politics and tech, so that tech can become the servant of progress and optimism, not a source of fear and suspicion.”
Reaction to Mr Clegg’s decision to take the Facebook role has been largely sceptical.
James Cleverly, deputy chairman of the Conservative Party, said it was “a good fit … But he shouldn’t pretend that it’s some form of noble, moral, higher calling.”
Labour deputy leader Tom Watson said Mr Clegg and Facebook had one thing in common: “They were both cool in 2010.”
Some, however, backed the former party leader’s move away from frontline politics.
Lib Dem John Cleese tweeted: “Nick is bright and decent and the kind of person the social media companies need.”
Publisher and LBC presenter Iain Dale said: “He’s still a youngish man and for people to criticise him for taking it says more about them than it does about him. He will be a loss to our political debate.”
Mr Clegg has previously criticised Facebook for its apparent approach to minimising taxes and a “grating” culture.
“I actually find the messianic Californian, new-worldy, touchy-feely culture of Facebook a little grating,” he said in 2016.
“Nor am I sure that companies such as Facebook really pay all the tax they could – though that’s as much the fault of governments who still haven’t got their tax act together.”
Register for free to continue reading
Registration is a free and easy way to support our truly independent journalism
By registering, you will also enjoy limited access to Premium articles, exclusive newsletters, commenting, and virtual events with our leading journalists
Already have an account? sign in
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies