In 1979 there was no internet. To gain access to the lists we dialed our modems onto local BBS networks. Making our own contributions and reading those of others, we quickly found how desperate some people were to see their names online no matter how excruciatingly banal their writings might be. Just as quickly we discovered how easily a tactless comment might cause offense and, by extension, how easily trollers could stir up trouble by artfully rubbing innocents the wrong way. And finally we found that spammers loved the system's lack of blocking tools. These problems mostly got sorted out as the system and its users matured so that these days the various forms of discussion-list systems are more popular than ever. Undoubtedly the most widely used of all newsgroup sites is craigslist. Although it's no longer based on Usenet software, it started as a discussion list and, even now, retains some of its ancient Usenet character.
Personally, I've found newsgroups best at accomplishing very specific tasks like sharing open source software, buying and selling bikes and bike parts, or finding answers to arcane questions such as the diagnosis of carburation problems in the 1979 Triumph Spitfire. I've also found genealogy lists to be extensive and quite helpful. And lately I've noticed that there's both longevity and considerable success in the running of hyper-local discussion lists — those newsgroups, email lists, and forums that have active moderation, require registration, and confine themselves to a relatively small geographic area as well as (usually) a defined subject-area of coverage. This link, for example, takes you to a set of discussion lists in and around a the section of Washington, DC, called Brookland: Links to listservs in Brookland and beyond. There are many such. Neighborhoods have discussion lists, schools have them, and many, many other local affinity groups have them. I've found, for example that the aging in place movement supports a growing list of discussion groups. I sometimes follow the one in my area called the Chevy Chase At Home Urban Village.
It'll be interesting to see whether the growing popularity of hand-held internet devices dents the continuing expansion of discussion list usage. It's tempting to guess that it will because the discussions are all keyboard generated and the pocketable internet appears mainly to be swipe-and-tap. Still, on the other hand, think how much text messaging there's been among cell phone users and how much more common it's becoming to find screen-based keyboards on tablets and other portables. I'm thinking the three decades of discussion list growth haven't brought this technology to the end of the line; we may find that the service begun when Usenet was born hasn't yet completed its cycle of usefulness.
Internet: Many-to-Many Communication
Bulletin board system
What Is The Usenet? from the Usenet Learning Center
What is Usenet? from FAQs.org
Usenet: the Global Watering Hole (1991)
List of newsgroups
How the Usenet News Protocols Work
Forum Software Timeline 1994 - 2010
List of Internet forums
DCist: What's Your Neighborhood Listserv? 2005
 Data source: Internet forum in wikipedia
 As this graphic from altopia (via wikipedia) shows.
An 2009 article in Wired says the Google Usenet Newsgroup archive contains more than 700 million articles. (See also this one.) According to CataList, the official catalog of LISTSERV lists, there are this month 515,217 LISTSERV lists, 2,773 of which have 1,000 users or more and 264 of which have 10,000 users or more
 This graph shows traffic on Giganews, a major Usenet platform.
 Regarding craigslist popularity, examine this comparison of Amazon, eBay, and craigslist traffic over the past few years.
These days craigslist has well over 815 million page loads a day according to doloop.com. According to wikipedia, craigslist "serves over twenty billion page views per month, putting it in 33rd place overall among web sites worldwide and 7th place overall among web sites in the United States (per Alexa.com on June 28, 2010), with over 49.4 million unique monthly visitors in the United States alone (per Compete.com on January 8, 2010)"
 In fact one of the more popular posts on my blog is one that shows the Spitfire's front wishbone suspension.
 See for example the lists on rootsweb and the Mailing Lists section of Cyndi's List.
 Of the 515,217 current discussion lists only 52,617 are "public." The rest are for defined groups whose members register and are approved by a moderator.
 Chevy Chase at Home says it's "a group of residents and friends dedicated to supporting the concept of Aging in Place. It consists of persons living in the Chevy Chase, MD communities of Section Three, Section Five, Martin's Addition, Chevy Chase Village and the Town of Chevy Chase who wish to help each other so they can maintain a quality of life they desire, and continue to live independently in their homes and community, enjoying fulfilling lives as they grow older."