I think it's a completely personal decision. Typically people wait until after the 12 week scan to tell so they know baby is in the right place, healthy, and growing well, but if someone's had a few early scans and all has been well before then they might decide to tell earlier. It's also down to whether or not someone would be able to cope with everybody knowing if something was/did go wrong in the early stages.
We announced on social media with DS around 10 weeks, after we had an early scan at 9 weeks and told OH's parents. I mainly couldn't hide it from work people anymore and I was impatient.
This time I'm now 9+5 and still haven't announced on social media. We are waiting to post it because one of OH's best friends is out of the country and she comes back the day after our 12 week scan and want to tell her in person. So it's looking like a Halloween themed announcement this time! I'm lucky though as I'm still off work and don't have a social life so no one sees me... otherwise they'd guess with how bloated and round I am already!
I've seen some people wait until their gender scan, others post as soon as they get a positive test, and some not say anything at all on social media! It's completely down to what you're comfortable with. x