By Sarah Young

LONDON (Reuters) - Facebook <FB.O> said it would expand its presence in Britain by 50 percent in 2017, joining other U.S. technology firms in increasing investment despite the uncertainty sparked by the country's vote to leave the European Union.

The social network firm said it would hire 500 new staff, adding to the 1,000 people it already employs in Britain, as Facebook gears up to open a new UK headquarters in London next year, following other firms drawn by talent and a thriving tech start-up scene.

Before the Brexit referendum in June, campaigners in favor of remaining in the EU had warned that international companies could seek to reduce their presence in Britain as a withdrawal from the bloc would make it a less attractive place to invest.

Some big banks such as Goldman Sachs <GS.N> and Citi <C.N>, which employ many thousands in London's financial center, are said to be considering shifting some jobs elsewhere in Europe as a result of Brexit, a worry for the British economy where financial services account for around 10 percent of output and provide some of the best paying jobs.

Facebook's UK expansion comes after Google <GOOGL.O>, owned by parent company Alphabet Inc, said earlier in November that it would invest an estimated 1 billion pounds, and make 3,000 new hires in the Britain.

"The UK is definitely one of the very best places to be a technology company," Facebook vice-president of Europe, Middle East and Africa Nicola Mendelsohn said at a conference run by the CBI, an employers group, adding that it was too early to say what Brexit would mean for the movement of labor.

"The movement of talent is something...that matters to us." she said, adding that Facebook employs people of 65 different nationalities in Britain.

Facebook opened its first engineering office outside the U.S. in London in 2012 and has been growing rapidly in the UK since. Its main UK operating unit's staffing increased by 90 percent in 2015 from 2014, according to its latest accounts, with more than half of those hired engineers.

It also employs hundreds of sales and marketing staff who tend to focus on larger clients or running regional teams, according to Facebook job ads and linkedIN profiles of staff.

Amazon <AMZN.O>, which created 3,500 UK jobs in 2016 at its head office, research and development centers, customer service centers and distribution depots, plans a further 2,300 jobs at three new distribution centers in 2017.

AMAZING ECOSYSTEM

Such investments have helped Britain's tech sector to shine at a time when some other firms are less optimistic.

A recent survey showed that three out of four companies with sales between 100 million pounds and 1 billion pounds have considered moving operations to the European continent in the wake of the vote.

Mendelsohn said that Britain needed to work hard to keep its position as a global hub for technology.

"We need to make sure that we continue to look outwards and not inwards; we need to stay competitive, and we need to remain a welcome home to tech," she said.

London ranked no.1 for start-ups in the 2016 European Digital City Index, and Google said the talent pool, educational institutions, and passion for innovation helped it decide to invest more in Britain, sentiments echoed by Mendelsohn.

"It's a place that our engineers want to come and work at, it's a place that we see this amazing ecosystem, not of just tech companies but also of creative companies coming together inspiring, fuelling one another," she said.

Facebook has a complex corporate structure for tax purposes and declared Dublin as its European headquarters which means almost no taxable profits are reported in the UK. It also has significant historic tax losses on its books, which means any cut in corporation tax would have little impact on its finances.

UK Prime Minister Theresa May is reported to be considering cutting corporation tax from the 20 percent headline rate in a move to attract companies away from other parts of the EU to Britain.

(Additional reporting by Tom Bergin; Editing by Kate Holton and Alexander Smith)