Hiyer,
You have many choices here, its a complicated question. Firstly who you tell and if you tell are your choice, and personal to you, you don’t have to tell people or you can tell people but don’t feel obliged in either direction because everything is ‘acceptable’. Some chose to just tell close friends and family only, some are selective in individuals outside that group and some just go for telling everyone as that feels right for them in their circumstances.
I personally told very few people (but I wasn’t working at the time, which can make a difference) as I felt I didn’t want to always be asked about my BC and wanted some time to feel normal without it as part of my conversation. However, some people feel better just getting it out of the way and out there. Having control of who you tell, when and how, is part of your personal right and need at this time.
If you want to let larger groups have knowledge of your medical position - you don’t need to personally talk to them. For example you could let someone at work know who is in a position to communicate to others and say that you don’t particularly want to discuss it with anyone, but you want to let people know the situation is ongoing. Or you can select certain people and ask that they don’t pass it on and share with others, OR you can ask that a few friends pass the information on to various other people. I was never one for using social media anyway, and would certainly not have been comfortable with making an announcement that way, however I’ve seen plenty of others do just that - so in essence do what you think is right for you. However, don’t feel anxious about how you decide to do this, or how many you decide to tell - there are no ‘social norms’ for this type of situation.
How to tell ? If there are people you want to let know outside your main social/support group then how much information you pass on is also your choice. You can even start off a conversation saying ‘Forgive me if I don’t really give you much detail, but I just wanted you to know that I have been diagnosed with…I don’t feel at this moment strong enough to go into details’. Its really hard because of course people will and do question, they want to show they care, and are interested and offer support, but I recognise that melting into an emotional state is something you fear. (We do need to recognise, that it is very hard for people to hit the mark when talking to a cancer patient, as there are such individual differences in what people need and can cope with.) Go for it fast or slow - neither is right and neither is wrong, pros and cons in either direction, so you do what feels right for you based on your circumstances.
(Edited to add - later in the process, after active treatment was finished, when I didn’t feel so emotionally vulnerable I was very free and open with who I commented to )
Happy to talk about this further…hope this might help a bit ?