What happens to your data when a platform shuts down?
It depends on why it closed and what the terms said, and the range of outcomes runs from an orderly export window to everything disappearing without warning.
A planned shutdown — the better case. Companies winding down deliberately usually announce a date, provide a data export tool, and allow a window of weeks or months to retrieve your content. Google has done this repeatedly with discontinued services. Even here, what you can export is often your content but not its context — you may get your posts without the comments, the social graph or anything others contributed.
An acquisition. Your data typically transfers to the acquiring company, which is explicitly permitted by most privacy policies — there is nearly always a clause allowing transfer of personal data as a business asset in a merger or sale. The new owner may then change the terms. You consented to this when you joined, and almost nobody reads that clause.
Insolvency. The worst case. In an administration or liquidation, user data is an asset that may be sold to creditors' benefit, and the priority is realising value rather than protecting users. Servers can also simply be switched off when bills go unpaid, sometimes with no notice at all. Several services have vanished this way.
What you can do in advance:
Export regularly. Most platforms offer a data download under GDPR data portability obligations. Do it periodically, not when a closure is announced.
Keep originals. Do not treat a platform as your only copy of photos, writing or video. This is the single most effective protection.
Own the important things. Content on a domain and an email list you control cannot be switched off by someone else's business decision.
Note what cannot be exported — follower relationships, a username, an audience, and reputation are platform-specific and not portable.
Archives such as the Internet Archive sometimes preserve public material, but not private accounts.