Mathematics for Web 2.0, Mathfuture Wiki.

#mathchat, a Twitter hashtag, by Maria Droujkova.
#mathmarkup, a twitter hashtag for LaTeX, MathML and related topics.

The Mathematics24x7.ning.com social network, created by Rashmi Kathuria on the Ning.com service.

The LinkedIn.com, discussion group Math, Math Education, Math Culture, created by Opher Liba.

The LinkedIn.com, discussion group Math Matters , created by Rebecca Hanson.
Wiki used for sharing diagrams to the above LinkedIn group http://linkedinmathmatters.wikispaces.com/

Orkut Mathematics (One), owner: Wat An Idea.

Orkut Mathematics (Two), moderators: Balakrishnan, Mihir, Hrishikesh, Ranajeet, Cláudio, Paramanand, NiLos.

Mathematics CyberBoard http://www.sosmath.com/CBB/ include equations that are entered as LaTeX and embedded as images within the text.

The Thesaurus of mathematical terms http://thesaurus.maths.org/ uses Java for graphics. Browsing the site with Firefox, it uses MathML for equations. Browsing with Internet Explorer, it requires the free MathPlayer plugin; as described on the the Help Page.