"It's been amazing to see how people have used Facebook to build a real community and help each other in so many ways," said Zuckerberg in a rare comment for the anniversary. But has it created a real Jewish community, or just made it easier for us to feel we are living a Jewish life - posting a photograph of our Purim costume, or wishing friends a good fast - without actually doing anything meaningful?
Arguing for the former, Jo Hyams of Tikun says she finds it indispensible for co-ordinating its social action projects. "It's amazing that so much Jewish life is online loud and proud! How far we have come."
Certainly, beyond the grating "off to Israel for Pesach" updates, the site has proved an unrivalled tool for organisations that rely on getting the word out to a wide audience. "Facebook gives us a much wider reach," says UJIA marketing manager Clare Gaba, who runs the organisation's Facebook page. "This is especially true for Young UJIA, but applies across the board. We can spread the message about our work in the UK and the Galil, let participants know how much they've raised and promote events."
As she points out: "There are not many people I know who don't surf the net at least once a day. If we want to get in touch with our audience, Facebook is an excellent platform."
Gone are the days when Jewish groups had to stuff envelopes, hoping that address lists were up to date. Campaigns now rely on the fact that information about a project can make its way on to a user's Facebook newsfeed instantaneously - and for free. "It has the potential for campaigns that may have remained fringe to gain traction quickly," says UJS executive director David Brown. Thanks to Facebook, schemes like the UJS's "liberation" networks, seeking to provide more representation for female, disabled and LGBT students, "gained attention quicker and from people who usually would not have come across these issues".
Jewish life has to an extent always relied on social networks - shul groups, youth movements and the like - but it was often a struggle to bring in those outside the fold. Facebook, by nature, is about expanding a community - ideal for university J-Socs engaging with students from outside the bubble, or the organisers of alternative minyans and grassroots projects.
"Last year UJS was contacted by Jewish students who wanted to set up a J-Soc at Plymouth," Brown recalls. They put out a call on Facebook and "within minutes", a combination of tagging, likes and comments indentified a number of Jewish students.
It's very helpful in engaging the Jewishly-interested but time-poor. "The simpler something is, the more likely they are to get involved," says Tikun's Hyams. But she admits this can be an illusion. Clicking on "attending" does not necessarily mean hundreds of people join every social action project. "People perhaps don't always take it as seriously. We are bombarded with invites on Facebook and don't always take any notice." And even if we do, what next? "After last year's Young UJIA Purim party, loads of people posted pictures from the photo booth - it created a really nice feeling," says Gaba. But is that sort of activity really enough to build a Jewish identity from? Perhaps not. Still, most would argue that the benefits of social networking online outweigh the dangers of it becoming a substitute for offline Jewish life. More worrying, say critics, is the potential trivialising of big issues. CST's Mark Gardner argues that when it comes to serious debate, Facebook allows control of the message "without it being shortened by journalists or given misleading headlines".
But "liking" a status is surely no match for attending a rally, or questioning decision-makers. In any case, is Facebook really the place to engage in nuanced debate about Israel? "Given the seriousness of the conflict, sometimes the casual, snappy nature of social media feels woefully unsuited," Brown concedes.
And, inevitably, since the early years, Facebook has been a breeding ground for the spread of misinformation and antisemitism - much of which survives unchallenged. Speaking recently, Facebook's Mendelsohn emphasised: "We don't want to have a platform where people share things for the sake of sharing gruesome horrible things, that's not the point." But critics are sceptical about how much Facebook acts on this.
"For such a huge company, their investment in moderations staff, policies and training seems quite poor," Gardner says. "Nobody elected Facebook to decide our local laws on incitement. Just because something is on Facebook should not render it immune to prosecution. The 'ritual murder page' is probably illegal under UK law and has not been removed."
Admittedly, much of this activity has moved sideways on to Twitter. Yet for all the predictions of its demise, Facebook now has 1.2 billion active monthly users spanning the generations.
At weddings, chupah photos are on Facebook before the starters are served. Rabbis regularly have Facebook conversations with their congregants. UK Jews can communicate with others around the world on a daily basis in a way that was once unimaginable.
"People tend to concentrate on the negatives," says Gardner. "But look at how it encourages people to talk about their pride in being Jewish. In fact, "it is a great shame that more has not been done to explore these positive reinforcements of Jewish identity". By the time Facebook celebrates its barmitzvah, maybe we'll be some way to making that happen.
