Farewell from Dave Lee - the BBC's man in Silicon Valley

Today, I end what has been four and a half years as the BBC's North America technology reporter. Or, as our presenters mostly put it: "The BBC's man in Silicon Valley."

When I started, I predicted my time here would be spent witnessing the arrival of the next big thing, whatever on Earth (or beyond) that happened to be.

It was a time of peak unicorn: a flurry of companies reaching that all-important - though arbitrary - $1bn valuation.

Many told me I would imminently be covering another burst of the tech bubble.

I have to say, I was always more optimistic: this was not like the 90s "dot-com bubble". It still isn't.

Instead, I have ended up covering something quite, quite different.

Change of focus

While the first half of the tech decade was shaped by things announced during keynote addresses, the latter half's news came not from a stage, but from a platform.

Women using social media to stand up against misogyny and abuse at Uber; activists using public protests to hit back at Google's military work; whistleblowers turning to the media to expose privacy violations at Facebook - to name but a few.

It was a change of focus that took some getting used to.

I'd find myself reporting on Google's latest smartphone one day, and then reporting on the firm's cover-up of an executive's sexual misconduct the next.

I'd hear in my ear presenters cueing me in to live coverage with the phrase, "And here to make sense of all that...", and I would wonder where to possibly begin.

It is clear that whoever inherits this role - to be announced in the new year - will need to navigate carefully the next evolution of these important stories.

It will be an era when the tech giants may be forced to atone for the prior decade's sins.

Early in 2020, the US trade regulator is expected to put in place measures to prevent Facebook going ahead with plans to tightly integrate its "family" of apps.

The signalling from regulators is clear: do not try to stop us breaking you up.

The ensuing battle will be fascinating. Do not expect Silicon Valley to roll over.

Digital sweatshop

It will also be a decade when the impact of running these enormous networks is placed under close scrutiny.

Recent investigations by The Verge have detailed the devastating impact that moderating harmful material has on the people unfortunate enough to have to do the job, for a pittance.

The "digital sweatshop" is here; it's ugly, and we need to start coming to terms with what that means.

If I was to make an ultimate prediction, though, I'd say the defining tech story for the next five years will be the growing chasm between the West and China.

In technology terms, this could mean a literal separation of the internet, with the US and its allies eschewing Chinese-made networking tech on the grounds of national security.

That's bad news for anyone who, like myself, still holds on to the optimism that the internet, at its purest, should make the world smaller - and safer.

Differing views on personal privacy are likely to lead to China stealing a march on the West when it comes to artificial intelligence, though that may be a price we're willing to pay.

Or maybe not. We'll see.

I'll continue to write about these issues in my new role. But as for this corner of the web, I'm signing off.

I hope my work during this time has gone at least some way to dissecting what has been happening here. It has been an incredible privilege.

Dave Lee, BBC News, San Francisco.