Many-to-Many Space : Social Software Timeline

1962

Douglas Engelbart writes Augmenting Human Intellect paper
J.C.R. Licklider & W. Clark, MIT: On-Line Man Computer Communication

1971

Ray Tomlinson invents email program

1973

Doug Brown writes the Talkomatic group chat program for the PLATO system.

David Woolley writes PLATO Notes, an online forum/message board system. It later inspires adaptations to many other platforms, including the "notesfiles" program that became part of Usenet in the late 1970's, and Ray Ozzie's Lotus Notes in the 1980's.

1975

Steve Walker creates the first ARPANET mailing list, MsgGroup.

Robert Parnes writes the computer conferencing system CONFER for the Michigan Terminal System. Jan Wolter's Partial History of Computer Conferencing in Ann Arbor traces its path to The Well's Picospan presentation skills(see 1985 below)

1978

Roy Trubshaw writes the very first Multi-User Dungeon (MUD)

wrong. MUDs were already up and running on PLATO several years earlier.
sorry, this is right, and widely documented.
http://www.ludd.luth.se/mud/aber/mud-history.html and elsewhere
http://www.livinginternet.com/d/di_major.htm
the reference to Plato is probably to 'Avatar', a Dungeons and Dragons style game, one of several that inhabited the Plato system - however the development of Avatar appears to have come after the development of MUD1 - http://www.youhaventlived.com/qblog/2006/QBlog010206A.html
It may also refer to _Oubliette, _released November 18, 1977 on PLATO - however this is not a MUD proper but rather belongs to the set of precursors, including Adventure (1972) and Mazewar (1973). See more here http://terranova.blogs.com/terra_nova/2005/11/news_miscellany.html There is a huge difference between games that run on one system, as the Adventure and Plato games do, and games that run on multiple systems, as the MUDs do.
Here's another timeline - http://www.raphkoster.com/gaming/mudtimeline.shtml
-- Stephen

1979

Tom Truscott and Jim Ellis create USENET newsgroups _(further reading: Brad Templeton, I remember USENET, and the USENET 20 Year Timeline)_

1984

Birth of the Fido network of Bulletin Board Systems (BBSes)

1985

Whole Earth 'Lectronic Link (WELL) community gets started

1988

August - Jarkko Oikarinen creates Internet Relay Chat (IRC) (Timeline)

1989

Peter Deutsch at McGill University creates "archie", an index of global file archive (FTP) sites

1990

Chip Morningstar: Lessons from Lucasfilm's Habitat

1991

Tim Berners-Lee posts World-Wide Web: Executive Summary to USENET group alt.hypertext
Mark McCahill at University of Minnesota releases "gopher", first simple menu-driven client to Internet resources

1992

January - WWW founder Tim Berners-Lee creates his What's New? page, arguably the first weblog.

1993

Howard Rheingold: The Virtual Community
March - Marc Andreessen announces the Mosaic web browser

1994

January - "Christ is coming" - first spam on USENET
April - Jim Clark and Marc Andreessen found Mosaic Communications (which will be renamed to Netscape in November)

1995

March -

Yahoo! is incorporated
Ward Cunningham's wiki launches (the first wiki)

December - AltaVista Web search engine launches

1996

January - 100,000 WWW servers

1997

April - 1,000,000 WWW servers
September - Rob Malda's Slashdot, the first weblog to enable reader comments (to check), goes online
December - Jorn Barger coins the term "Weblog"

1998

June - DMOZ, the open directory project, is founded by Rich Skrenta and Bob Truel under the name GnuHoo
September - Google is incorporated
October - NewHoo (nee Gnuhoo) acquired by Netscape, and renamed directory.mozilla.org DMOZ for short.

1999

Peter Merholz coins the term "blog"
March - Brad Fitzpatrick's LiveJournal goes live (Timeline)
June - Rob Malda sells Slashdot to Andover.net
June - Netscape Search becomes first major portal to incorporate Google into its Search results
July -

RSS 0.91 specification
Pitas launches

August - Pyra Labs' Blogger launches
November - Userland's Manila launches
December - Rusty Foster creates kuro5hin, a weblog where users vote for what goes to the front page (Timeline)
Shawn Fanning creates Napster
Match.com acquired by Ticketmaster-CitySearch

2000

August - RSS 1.0 specification
October - James Hong founds HotOrNot.com with zero capital

2001

January - Larry Sanger and Jimbo Wales create Wikipedia (Timeline)
July - Cameron Marlow's Blogdex Kenali dan Kunjungi Objek Wisata di Pandeglang launches
September - Movable Type initial beta release
October - Adrian Scott's Ryze social networking service launches

2002

January - Userland releases the Radio Userland blogging software
February -

"10,000,000th WWW server goes live
10,000,000th post on Blogger

March - Spoke Software founders Jim McDermott, Ben Smith, Paul Reddy and Chris Tolles incubate initial prototype at USVP
April - Jonathan Abrams' Friendster launches
December - Socialtext is founded

2003

January - Wikipedia hits 100,000 articles
February - Blogger acquired by Google
March - hundreds of people show up at Dean Meetup in New York City
April -

Many-to-Many launches
LiveJournal hits 1 million accounts
Social Software Alliance forms

June - Flash mobs begin in Manhattan
July -

Deanspace launches
Friendster passes 1 million accounts

August - First Flash Mob in Britain(London)
September - Sharman Networks releases Skype
October - First Flash Mob in Bombay

2004

January - LiveJournal hits 2 million accounts
April -

Skype hits 10 million downloads
The YASNS lists 209 different social networking systems.

References

Living Internet
Net.History
Timeline of the History of Information by G. Nunberg - provides a bit of perspective on things!