I've told tons of people. My parents, siblings, close friends, spouse's parents, people at school who needed to know for academic reasons, etc. I haven't said anything on social media so those who I don't see in person don't know yet (I will probably say something the day or two before surgery). I've had mostly positive reactions, with a few mixed reactions as well. My MIL said, "Are you sure?" First thing out of her mouth. But she's gotten much more supportive since then. My best friend was concerned about my safety and the risks of surgery. We spent a while talking about the surgery and recovery time and she felt better after that.
I only had one reaction that really upset me, which was another close friend who responded with, "What do your parents think about it?" and then changed the subject. This is my best friend from childhood so I definitely was not expecting that kind of reaction. She was the first person I told so I got scared from telling anyone else and didn't mention it again for several months. She's been very supportive recently, though. Maybe she just needed time to process it.
I do have to say that none of my family members or close friends is obese (my parents are overweight, though). So I haven't had to deal with the potentially jealous or defensive reactions. I do have one obese friend whom I have specifically not told yet... in the past she has made comments making fun of skinny people, to make herself feel better I think. I think she sees us as "in this together" and also may be thankful that I am bigger than her so she doesn't have to be the biggest one in the room, if that makes sense? Of everyone I think she's the most likely to react negatively, so I've been waiting until the right time to mention it.
So... all of that said. I think if you want to tell people you should, but bear in mind their own insecurities about weight. It's likely that their reaction has more to do with them than with you. The more you know about your surgery, the better the reactions will probably be. You can answer questions and show them you've researched for quite some time and you're not just looking for a magic bullet.
I'm personally not a fan of keeping it secret, I think it perpetuates the stigma against WLS, but it's a personal decision and depends on how you feel about advocating for yourself if someone should speak against you or the surgery. So I really don't blame anyone who chooses not to tell.