Here's an interesting overview of a new hacking issue, and its impacts, at Twitter HQ.
Late last week, Twitter CEO Jack Dorsey had his Twitter account (@Jack) hacked by a group who call themselves 'Chuckling Squad'.

The hackers subsequently tweeted out a range of racist comments, before Twitter was able to regain control of the account. In the wash-up, Twitter identified that the root cause originated from Dorsey's mobile provider, with the hackers using a process which reportedly involves swapping SIM cards.
The phone number associated with the account was compromised due to a security oversight by the mobile provider. This allowed an unauthorized person to compose and send tweets via text message from the phone number. That issue is now resolved.
— Twitter Comms (@TwitterComms) August 31, 2019
The good news for Twitter in that case is that its internal systems didn't fail, and the hackers couldn't access more sensitive information in Dorsey's DMs. But it still highlighted a significant flaw, and a concern Twitter needs to address in order to avoid the same happening to Dorsey, or others, in future.
As such, as it investigates how to fix the issue, Twitter earlier this week announced that it would be switching off the ability to tweet via SMS entirely.
We’re temporarily turning off the ability to Tweet via SMS, or text message, to protect people’s accounts.
— Twitter Support (@TwitterSupport) September 4, 2019
But that's not ideal either, as many developing regions rely on tweeting via SMS to use the service. Consequently, Twitter has now reinstated the option in certain usage areas.
We’ve now turned this feature back on for a few locations that depend on SMS to Tweet. It remains turned off for the rest of the world.
— Twitter Support (@TwitterSupport) September 5, 2019
But the problem itself is not fixed - and as of right now, most people still cannot tweet via SMS.
That may sound like a major deal, but for context, back in 2015, Twitter announced that it would start including what it called "SMS Fast Followers" - or people who sign up and access Twitter entirely using text messages - in its monthly active user stats, which, at that time, equated to an extra 6.4 million users.

As per Quartz:
"Looking at the last five quarters [as of April 2015], the inclusion of SMS Fast Followers would have increased Twitter’s monthly active user count by 1% to 2% each quarter."
2% of all Twitter users is significant, especially for a company that's constantly working to boost its user count.
So while it may seem like a fairly limited consideration - and in broader context, it is - based on 2015 numbers, the change, when announced earlier in the week, would have meant that close to 3 million current daily active users weren't able to use the platform at all, up until Twitter re-instated the option yesterday.
So, as a note of context, things are, as always, a little confusing at Twitter. And it hasn't been able to resolve the SIM hack issue as yet.