Guestbooks aren't too useful anymore as most of what they do, social media does better.
Usage
Guestbooks were definitely a thing, GuestWorld alone had 1.1 million users in 1998, when it was bought for 3.9 million USD.
Guestbooks are a feature of "web 2.0" still, MediaWiki, DeviantArt, Steam and others still have it in the form of "profile comments" or user talk pages. YouTube used to have channel comments as well, before they first got hidden in a tab and then removed.
Conversations
Compared to social media, getting a conversation going on a guestbook or profile comment is awkward:
On MediaWiki, the way conversations in user talk pages work is that Alice would write a message on Bob's user talk page, which triggers a notification bar when Bob browses the wiki:
If Bob decides to respond, he could either
- do it on his own talk page, in which case Alice better have put his talk page on her watch list and sign up to updates for all correspondence on Bob's talk page (this is Wikipedia's method), or
- he could reply on Alice's talk page which would trigger her orange banner, but make following the conversation much more difficult (this is the method my wiki used).
Note that according to the people who bought GuestWorld, this janky public communicating was the main purpose of guestbooks:
As guestbooks foster two-way communication between site builders and visitors, we expect these services will encourage repeat traffic.
Modern social media, particularly twitter-inspired ones, solve this use case with a lot less jank.
Spam
A second issue with guestbooks in particular is spam. Looking at various old pages with archived guestbooks, many of them feature some variation of
I've had to shut down the guestbook form that was previously on this page, due to concerns about spammers hijacking the form script. If you'd like to contact me, please send an email.
I run some very unknown wordpress blogs, and the ratio of spam comments on posts to anything genuine is somewhere in the order of 1000:1. The anti-spam tools of today are capable of filtering most of it out of course, but at the tail end of guestbooks as a common thing, not so much. YouTube to this date doesn't allow the characters < and > in video descriptions because in 2005, this was how you'd protect yourself against XSS attacks. It's only thanks to the forced switch to Google+ and then away from it again that YouTube comments now can use these characters.
Comments on content are somewhat fading away for a similar reason: Between a polarized public and Russian troll farms, the comment sections of newspapers in particular have turned so vile that a range of publications had to introduce opening hours for their comment sections as to not overwhelm their moderators, or close comments altogether.
Using social media offloads the moderation requirement onto a third party. And even though moderation on all of the large social media platforms is notoriously bad and unfair, their anti-"scaled abuse" teams generally do a decent job to not allow blatant spammers overrun the site.
Reviews and testimonials
Profile comments and guestbooks operate on the presumption that the commenter has checked out large parts of the site before writing the entry. But many sites today, including personal blogs, are far too large for this to make sense. Taking a random Geocities sampling (via oocities), it seems like websites used to be an "about me" + a handful of pages on a certain subject. Given that you had to do all of the HTML yourself, that's reasonably difficult.
With blogging software like wordpress and blogger though, a lot of the overhead is removed and it's feasible to publish lots more content, and crucially, publish lots more timely content which is relevant for two weeks exactly. With search engines becoming good, it then means that visitors will read the one article they were given via search, before wanting to post their comment on it somewhere.
A guestbook here is a bit janky again; given that it lacks the content on what you're commenting on, it's clearly inferior to a per-article comment section.