I never joined Facebook. I had the opportunity in 2004 when GaTech was one of the earlier schools for expansion beyond Harvard. They hired a promoter to spread the word on campus and I worked with her on the student newspaper. The MeFi "you aren't the customer, you're the product being sold" had been around a couple years, I recognized the pattern, and figured they'd be selling the personal information people gave them.
As to the "more innocent internet", think that social networks in general are an attempt to take the positive energy of the blogosphere and bottle it in their own walled garden. The result is messier than the inspiration due to the normal community degradation when a medium moves from early adopters into the mainstream.
Not all early adopter communities are great but being great is pretty highly correlated with success. When you start, you tend to have a bunch of intelligent and intellectually curious people and as long as everybody's civil, you tend to get a pretty good set of converstaions going. As long as growth is slow the community can assimilate newcomers and force them into its norms and things remain largely good. Once the rate of growth becomes too high, the norms are lost and people bring their own norms. Invariably some subset of the community becomes aggressive / belligerent, the opposition / moderates decide they don't have enough time to deal with that, and the community degrades into "normal" online discourse.
You can see this pattern from very early in the life of the internet. I wasn't around for it but I've heard the phrase "the September that never ended" show up in context of AOL bringing millions of people to usenet and overwhelming their ability to absorb newcomers, which traditionally had been in September when new students arrive at university and got online. I've seen similar patterns in Reddit, Hacker News, Twitter, and the Python and Node.js ecosystems. It can happen without the growth. HN is pretty negative despite being a smaller community. Reddit is interesting in that each subreddit seems to have its own lifecycle in this pattern so some are really nice (e.g. /r/rust) while others are completely toxic and the ones where everybody intersects (e.g. /r/politics) are the expected dumpster fires.
The only way I know of to guard against community degradation is heavy handed moderation. As far as I know, MetaFilter is still going and I've heard pretty good. The ArsTechnica forums, where I've been active for just under 20 years (makes me feel old) are have also maintained a consistent quality over time. I've talked to the Cake guys about this and will be interested in seeing how this platform develops over time.