This indicates a key vulnerability for the social behemoth, and failed attempts to address it reflect a point I made in a recent column: There’s no guarantee that Facebook’s current winning streak can last.

Facebook’s decline in personal updates reflects a common growing pain for online communities. What starts out as as special and intimate place to share things grows into a big, impersonal, and professional platform. Some online communities try to preserve the special and intimate at the expense of adding new users. (Consider communities like Reddit: thriving, but never quite crossing over into the mainstream.) Others crumble once they do reach the mainstream, causing users to abandon the service for the latest new thing that feels more special and intimate. (There are a litany of examples: Myspace, Bebo, Flickr, Orkut, LiveJournal, Friendster.)

Facebook is remarkable in that it has managed to avoid either path. It went mainstream but didn’t lose its appeal, because even if it lost that special and intimate feeling, it has become an essential utility for keeping up with friends and family. Facebook is still the first place where people are compelled to share meaningful updates like engagement announcements, baby photos, and vacation photos. A home for your personal press releases.

Which is why the decline in meaningful personal updates is a big problem. I believe there are two things driving the decline. First, there’s the rise of professional content on Facebook. Second, there’s the shift from content published in private to content published in public.

The increase in professional content on Facebook has been gradual, but the company has welcomed it. Facebook wants people to share this stuff—videos, news articles, entertainment—on its platform because it means people will spend more time inside its “walled garden” than anywhere else on the Internet. The company has done a great job of curating professional content and giving users what they like, and as a result, Facebook is more addictive than ever. Metrics like user engagement and time spent using the service have continued to rise. Even as analysts fret that Facebook is running out of Internet users to add, the company is turning millions of monthly active users into daily active users. (Last quarter 65% of Facebook’s monthly active users came back every single day.)

One little problem: Professional content can be found anywhere online. It wouldn’t be that difficult for a competitor to steal users away with a better, more addicting news app, or a better, more addicting way to watch viral videos, or a new, more addicting way to consume entertainment. Beyond that, all those inside jokes, blurry photos, and half-baked opinions you used to Post now feel out-of-place amid all the professional content.

On the other hand, personal updates—including the half-based opinions, but also the baby photos, engagement announcements, and vacation photos—are what keep people coming back to Facebook. It’s unlikely that users will get that information anywhere else, and they don’t want to miss important life updates from their friends and family. Without the personal updates, Facebook becomes a glorified, $327 billion content recommendation engine.

Facebook’s shift from content published in private to public has been just as subtle. There’s a reason the once-daily outrages over Facebook privacy scares have stopped. The platform no longer feels like an intimate conversation among friends, so users no longer expect full privacy. By default, users now expect that items posted to Facebook are done so in public. They know that, unlike in Facebook’s earlier days, their status updates can now be seen by distant relatives, high school classmates, and co-workers—so they don’t share anything too personal.

Besides, Facebook’s privacy settings are too complicated for most people to figure out, and everyone knows private updates can be easily screen-captured and shared. There’s always the risk that something could go viral and cost the person who posted it their job or worse. (Remember the death and rape threats received by charity worker Lindsey Stone’s offensive, but ultimately harmless, inside joke at Arlington Cemetery?)

The bar for what Facebook users once posted on the site used to be much lower, as BuzzFeed writer Alex Kantrowitz points out. A new feature showing old posts from years past only serves as a reminder of that fact.

One of the most interesting things about Facebook's 'On This Day' feature is it reminds you how much lower the threshold to post used to be

— Alex Kantrowitz (@Kantrowitz) April 7, 2016

The decline in personal sharing on Facebook is an important reminder that, even though Facebook completely dominates the social media market, its future domination is all but guaranteed. The more sophisticated the platform becomes, the more pressure the platform faces to retain a tiny bit of what first made it special: intimacy.