I'm not arguing that 2005 was in any way the beginning of the internet, but it was definitely the beginning of another epoch - Facebook, World of Warcraft, and YouTube were all brand new, and we were just two years away from the smartphone. Did online games, social media, and video sites exist before then? Oh yeah, absolutely, but things were becoming supercharged and life eating.
There's little doubt that we could use some standardized terms for different eras of the Internet, but I would characterize Facebook and YouTube as harbingers of the end of the frontier days.
I don't think "harbinger" is the right word here, I would reserve that for dot-bomb companies and services like myspace. That time (2000ish) had some pretty clear signals where things were going, while also being clear that it wasn't worked out yet.
By the time Facebook/YouTube/etc. showed up, the frontier was already a memory, and it was more a question of see who the big winners were going to be.
They kind of were. You can think of every new wave of organisational fashion as the new frontier days.
Think like this - first you had dial-in BBSes. Those had frontier days, peak and fall.
After BBSes came Usenet and IRC. Same trajectory. Both are almost dead.
After that we had PhpBB &al. Internet reimplementations of BBSes. Now you didn't go to alt.rec.anime, you went to somethingawful.com and posted stuff that made you cringe in the years after you graduated high school. Or you made webpages on geocities full of scrolling blinking text and slow-loading gifs.
Then we started getting stuff like 4chan, facebook, twitter, tumblr. Very different from one another, but all appeared at about the same time, so I'm counting them as a new frontier.
Currently I'm suspecting that Discord will have the same place in people's hearts that IRC had back in the day. It's practically a very media-rich IRC clone with features geared towards 10-20 year olds who play video games, i.e. the exact group who would pine about the "good old days" in about another 10-20 years.
Basically, whatever technology happens to be young at the time you're in high school is the "frontier of the internet" for you. It may just be a collection of BBS webforums or an entire form of communication like IRC.
> These were the frontier days of the internet
I'm not sure 2005 qualifies as the frontier days of the Internet.