Fundsmith revealed the new buy in the February factsheet for the fund, which was highlighted by users of Wealth Manager's sister site Citywire Money, but declined to comment further.
Facebook is one of the so-called Fangs: Facebook, Amazon, Netflix and Google, the latter now formally a division of Alphabet. The high growth-tech businesses are seen as at the head of a new wave of tech stocks with high levels of potential price power.
Smith's acquisition of Facebook is his first since in US business software provider Intuit and is a relative rarity given his emphasis on minimising portfolio turnover by backing companies for the long-term.
Smith describes his three-step investment process as 'buy good companies; don't overpay; do nothing'. While he has kept portfolio turnover at low levels, he has typically bought or sold at least two companies every year since launching the fund in 2010.
His Facebook buy comes as the shares trade near all-time highs of $177 per share, up more than 360% since their flotation in 2012.
But strong earnings from the US social media giant and healthy estimates mean the shares are trading close to their 'cheapest' levels since floating.
The $177 share price places the company on a trailing price-earnings (PE) ratio of 28.7 times, while strong forecasts for next year's earnings place them on a forward PE of 22.9 times, a low they haven't reached since the tech sell-off following Donald Trump's election as US president at the end of 2016. It also contrasts with a rating of more than 50 times in early 2014.
That places the shares at a discount to the wider IT services and consulting sector in the US, which is trading at around 28 times forward earnings.
Amazon trades on nearly 150 times forward earnings, and Twitter (TWTR.N) nearly 50 times, according to Reuters estimates.
Smith's Facebook buy has also bolstered his fund's strong weighting to the technology sector, now accounting for more than a third of its portfolio.
Consumer staples, the area of the market with which Smith's investment philosophy is most commonly associated, is now only the third largest sector weighting, at just under a quarter of the fund.
Smith's Facebook friends
The managers have and at the end of September held 4.7% of their £6.3 billion FTSE 100-listed trust in the stock, although the company has since fallen out of their top 10 holdings.
The UK fund managers with the highest conviction holding in the stock are those running technology-focused funds. Jeremy Gleeson holds 7.8% of his £445 million AXA Framlingon Global Technology fund in Facebook, while Citywire A-rated Ben Rogoff holds 6.1% of his Polar Capital Technology (PCT) investment trust, and 4.6% of his Global Technology open-ended fund, in the stock.
Other managers with a broader remit are also strong backers, however. Kristian Heugh, Citywire AAA-rated manager of the Morgan Stanley Global Opportunity fund, the only global fund open to UK investors to have beaten Fundsmith over five years, has a 6.1% stake, according to Thomson Reuters data.
Smith has meanwhile sold his position in Dr Pepper Snapple (DPS.N), a position he has held since launching his flagship fund, after Keurig Green Mountain agreed to buy the soft drinks maker for €18.7 billion.
Since launching the fund in November 2010, shares in Dr Pepper Snapple have climbed more than 210%, even before the currently 2%-yielding shares' dividends are taken into account.