Disclaimer: Vegetarian here, slowly turning vegan as I think about it more
Disclaimer 2: I do NOT preach or post facebook things etc. If people ask, I discuss it, but always carefully and minimally as I don't want to sound judgey. Only a very few times have I 'argued', when the meat-eater has been spouting factually incorrect nonsense.
I used to be a staunch meat eater, never considered being a vegetarian, believed all the crap about needing meat etc, believed that 'one person can't make a difference', believed that not eating meat was stupid and nutritionally dangerous, believed that as long as the meat was free range there were no ethical issues, believed that meat wasn't as bad for the environment as people made out, etc etc.
Then over a few years I slowly came round to actually realising that I was kidding myself, and that with my academic background (biologist) and understanding of environmental science I couldn't justify my choices. The big thing for me was realising that actually I could be perfectly happy and healthy without meat, as I wasn't really eating much of it anyway. The facts are indisputable, they are not opinions.
Opinions come in when we decide whether we can justify ignoring the facts for our dietary choices, and I don't want to argue that- I never do, and I recognise that you can't deny someone's right to an opinion. You can, however, make the facts available to them. Shoving them down their throats is unpleasant, but sharing posts about veganism on facebook is in my opinion perfectly valid. I don't do it myself, and I know many many vegans who don't, but I have a few friends who do and they don't do it in a preachy way, they do it in a 'raising awareness' way.
To be honest, I think some people get upset because they are uncomfortable with the truth- I say that because I was that person once, and I tried everything to justify to myself why vegetarians/vegans were being ridiculous. Then I realised what I was doing and realised that actually those 'dietary choices' were becoming more popular and easy to switch to as we are becoming more aware of what is involved.
It's very easy to scroll past- if you don't want to think about what they are raising for discussion, scroll past. If they pounce on you at a BBQ and start demanding that you put your burger down that is of course different- but I don't believe that happens nearly as often as people make out! (Though I have had people demand that I eat a 'proper' burger...)