Doesn't make any sense if you look at provided data points. The IQ, test scores and sleep quality of people fell because they couldn't have anonymous flame wars on Facebook? The laws you imply happened in 2007-2008 were all introduced in the 2010s. Facebooks real name crackdown happened in 2014...
Echo chambers are not a recent phenomenon, but social media turbo charged it refining the engagement factor. The fact moderation by and large in social media platforms were able to utilise a report button and a script, rather than live human moderators was all thanks IMO to the changes as per 2006 (see reply to AJRF [OP], 2007 is when iirc I started to notice the stark changes to the rest of the discussion areas and occasion angry ex Facebook user. The near humanless moderation apart from the occasion cookie cutter, was particularly conducive for designing algorithms to exploit people, especially young impressionable people.
Additionally no one was really closely monitoring the sleep quality of those who frequented the net, or those who were really engaged (near omni present) on the net pre 2005, but I don't recall thinking the conversations had back then were anything as borked as some ... most of the crud in some social media areas. I'm left with a sense, sensibility was much higher a couple of decades ago ... or one knew that they were going to be called out on whatever they said if it was in error.
Of course the ultimate cause is that the Internet changed from this mysterious third place full information to where we live our private and public life, where there's an unending torrent of things that affect us.
Smartphones and low-latency high-bandwidth ubiquitous wireless networks with large enough data quotas are the vector that facilitated this.
The nature of content changed from funny image macros with silly cats to weaponized out-of-context news videos (and reports, and studies, and data). From mostly boring IRC chatrooms full of cold quasi-autistic greybeards (or sometimes neurotic drama queens and kings) enforcing their idiosyncratic rules that allowed fun little interactions that were collection worthy on now almost forgotten websites (bash and qdb) to public Facebook posts and now to private group chats.
All this in front of the backdrop of global economic growth in its downturn. (The upsides of the China shock - and of globalization, in general - already reaped and now we're left with the boiling resentment coming from those who feel they were defrauded, who are attracted to narratives that dish out blame to elites and everybody close and far away in space and time.)
Previous presidents, infamous and obscure international organizations, and of course vague shadowy groups and half of the population all at once. Beancounters, the MBAs, real estate developers, private equity, CEOs, big pharma, WHO, toxic masculinity, LMBTQAI+, MAGA, and of course the DNC that caused all this by not letting Bernie became the nominee.
When people grow up with expectations that they'll live better than their parents and then that doesn't happen, we don't take it well.
None of that happened in 2007. I'm not even saying the Internet didn't change, but you are both at least three years too early to make it ground zero for the overall societal change like seen in OP's data. I would even say it's the opposite the 2000s had a lot of vigilantism and political activism that started on the net. Key examples include WikiLeaks, the Pirate Party movement in Europe, and everything surrounding torrents. BarCamps and LAN parties grew exponentially. Most corporations hadn't started enshittification yet. Fringe sites still had hosters. 4chan launched, and Anonymous played games. This was the height of the blogosphere, the Creative Commons movement launched, and RSS was omnipresent. For the normal user, the Internet was probably the freest at the time. I don't know why you wanted to ignore that time and replace it with something that followed years later just to ramble on a forum...
I don't think 2007 was special, it's the usual gradual change (but likely a lot of logistic S-curves summed), but IMHO the change did happen around then.
And usually we see the highpoints of certain processes, like the skyscrapers that top out just before the market crash. (And rarely we hear about the unfinished ones, especially the ones left in drawers.)
With the Internet it feels that we had it in its romantic form in books and movies, then as it really arrived it turned into this really boring aspect of our lives, that nevertheless eat up all the better ones. (And we move less, eat worse, ruminate more on what we said and sent, worry about what we read and received.)
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Normal users joined Facebook not Google Reader. That's the whole point. (They have phones and Twitter and other bad hyper-palatable addictive platforms and whatever garbage apps their store gave them.)
