There are a lot of good reasons to block Meta, but for me there are two reasons I keep thinking about: the harmfulness of algorithmic content, and commercialization.
Algorithmic Content
Algorithmic content promoted by Meta and similar platforms has proven to be harmful time and again. I feel like every few months some new study comes out that shows how harmful sites like Instagram are for people, especially kids. Algorithmic content feeds are purpose built to encourage addictive behavior in their users, how many times have you heard people say something like "I meant to go on Instagram/TikTok/whatever for 5 minutes and then I looked up and hours had passed". Algorithmic feeds are powerful, they change the way people create content, and the way people consume it, and not for the better. People end up creating content that is algorithm friendly, not content that they truly want to make. It turns social media into a very performative space, where genuine interactions are hard to come by.
The new Meta platform will absolutely be powered by algorithmic feeds, and that will have an outsized impact on the Fediverse. It will change how people post on and off Meta's platform, because in the end, if Meta users can see and interact with it, then you're effectively posting on their platform. If Meta succeeds in bringing over a large portion of their Instagram users to their new platform we could easily see the Fediverse consisting mostly of Meta users. This would mean that users outside of Meta's platform could have their content go viral within the Meta platform. Once people realize (subjectively or not) that their posts are more popular when they post in a way that pleases the Meta algorithm, then they'll change the way they post. Historically Meta and similar companies have used their algorithms to push content that has high engagement, content with high engagement tends to be divisive. I predict that we will see a large uptick in transphobic content, and other content that aligns with the current moral panic that is happening in America. I do not want to participate in a community filled with that content, and I don't want my posts to pass through the same algorithm that promotes that content.
Commercialization
Meta, Twitter, Reddit, and other similar companies all make the majority of their money on ads. They've all been desperately trying to rely less on ads, but none have been able to even come close to having an alternative way of making money that can sustain their companies. So far the Fediverse has avoided the need for ads, and in general has been pretty anti-corporate, which as been nice. The closest thing I've seen to an ad on Mastodon is small artists promoting their online stores to sell prints of their work. On Twitter and Reddit ads are everywhere, and they do everything they can to make them blend in with normal posts. What is really annoying though, is that these ads tend to get a ton of engagement, so even if it doesn't get put in your feed because it was a promoted tweet, some dipshit may retweet, quote tweet, or reply to an ad, which then pushes it into your feed. I have seen countless promoted tweets that have hundreds of likes, retweets, and comments. What this means is that even if Meta has no direct way to push a promoted tweet into your feed on a non-meta platform, they can still put their thumbs on the scales in a way that gets ads to be liked, boosted, and replied to, which will increase their reach outside of Meta's platform.
The problem isn't just promoted tweets though. We know for a fact that Meta is reaching out to "creators and public figures", like Oprah and the Dalai Lama. Meta knows that Twitter shit the bed and scared off a lot of big name celebrities, but also a lot of brands. There is no doubt that Meta is actively pursuing big brands and doing what they can to get them to join their platform at launch. With an influx of brands coming to Meta's platform, we'll see them start to have sway in the Fediverse. On Twitter Wendy's has 3.8M followers, Burger King has 2M, Steak-umm has 214k, Tim Hortons has 657k. Wendy's doesn't need to pay to promote their content, they get hundreds of replies, thousands of favorites, and hundreds of retweets on most of their tweets.
We also know that part of the reason brands like these big platforms is because they're given extra tooling that gives them more in depth information about the people who follow and interact with them. They get to see the data that Meta is harvesting, and they get to use that data to better target us.
I recently had to help a family friend deal with a scam that originated on Facebook, and I learned quite a lot more about how bad Meta really is as a company. They really truly do not care about their users at all.
A family friend hadn't been using their Facebook account, and recently discovered that a hacker took over their account. We don't know how exactly this happened, or even when exactly it happened. She did a google search to find the support number for Facebook, but of course Facebook doesn't actually have a support number you can call. In fact, they essentially have no way to contact them. Scammers however, have realized that they can SEO their way to the top of search rankings when you search for a Facebook support number. Long story short, the scammers shared the shit out of her, and spooked her into handing over $400 in order to verify her identity (they said it would be refunded), as well as copies of her drivers license, and other sensitive personal information.
After helping her clean up the mess from that scam we also tried to help her gain access to her hacked account, but in her case it's effectively impossible. The hackers changed the email and phone number on the account, so there is no way to recover the account. They have several different sets of instructions for recovering an account, but for her nothing worked. One of the instructions implies that a friend or family member can initiate the recovery of your account, but all it does is take you to a special menu on their profile that takes you to a support page, which tells you a bunch of stuff that doesn't help (like using a phone number to recover your account). Basically all she can do at this point is have her husband contact Facebook and claim that she is unable to use her account anymore due to medical reasons, and they'll delete the account, however they may not even honor this request.