Cost was minimal if you dialed local boards in places where that was cheap/untimed. Expensive if long distance: but people used tricks like corporate PBXs and calling cards and blue boxes to reduce those expenses. That stuff was mostly 1980s. By the mid 1990s the developed world had largely shifted to digital telephone exchanges (Ericsson AXE) and the box tricks were relegated to the very obscure parts of the developing world, much of which skipped copper (and the problem of copper theft) and went straight to mobile.

If you had a computer and a modem, other entry barriers were minimal. Most boards required only the number and you could call in. Cool warez boards required invites and had passwords. Generally IIRC they would agree to a carrier at any speed so you would in practice be able to talk to them on old hardware, maybe down to 2400bps.

The experience was more like a secret group of friends than a public social media network. It was generally local-ish, but not hyper-local, so you perhaps still had a chance to meet people IRL/AFK if desired. First, the sysop would be responsible for creating an environment in which people actually wanted to communicate. This would occur through selection of content, community and policy. Policies might be how long you could stay online per day, that sort of thing. Recall that each board had a limited number of lines and each required a telephone line monthly payment plus a modem plus a spare serial port. There were these ungodly serial ("COM port") expander cards with chains of tens of plugs... forget what they were called. If your friend came on the board you could ask to chat to them kind of like the unix 'talk' command. The codebases were fairly heterogeneous however, many sysops implemented or modified their own codebases. Probably a lot of them had buffer overflows.

As you were your own entertainment and your experience somehow timed, perhaps interactions tended toward an more open intellectualism missing from general society. The sort of thing HN sort-of-kind-of-has but not really. If HN were a BBS, it'd be effectively anonymous, effectively unsearchable (the search experience used to be a lot like manually trawling through a library card-file), and all the good articles would be downloadable with ASCII art and group greets. In the early popular internet era I remember l0pht ran a BBS for a bit but AFAIK it never really took off.

Can't comment on what the industry size was, I guess there were tens of thousands of boards and it was really an offshoot of 1980s computing culture where kids were reaching out to connect with others in an ever-growing subculture that was powerfully disjunct from the rest of society and thus liberating. Years later I found out I had a distant relative who was lesbian and ran a board for queers at this time, I guess much like the internet today a lot of people flocked to BBSs who felt marginalized by the rest of society.

Tech-wise, this was the era of UUCP/FidoNet, warez and batch-transfers. The initial popular commodification of digital information and software. The initial popularization of node-based addressing schemes. It was laying the groundwork for the popularization of the internet, in cultural terms, technical terms and communications infrastructure terms.

What made it to the internet? Initially, everything. Everyone online came from BBSs in the early days. But now .. it's hard to point to anything in particular. Maybe open source, a sort of seeping and perverted globalism of consumption (whereas then we had an excited and optimistic utopian global village) - basically the biggest change is everything - here included - is now commercial. It's a sad testament to human character, despite ever-crashing costs.

PS. I wrote a history of the modem at the end of a thread the other day, consequently nobody read it, but this thread's readers may enjoy: https://news.ycombinator.com/item?id=29052827