In online communities, the term is used to compare the volume of worthwhile information (signal) to the volume of redundant, irrelevant, and administrative material (noise). For example, in a Perl newsgroup, signal is useful answers to your Perl questions or other on-topic discussion, and noise is spam, first posts, "Natalie Portman Naked and Petrified", "unsubcibe" messages and other things that do not further your purpose in reading and participating in the newsgroup.
See OrderChaos for ways to increase the SignalToNoiseRatio.
Another way the SignalToNoiseRatio (also called S/N ratio) is useful in an online context is to describe the effects of various filtering methods. For instance, the SlashDot moderation/threshold system is remarkably effective in eliminating the noise from discussion groups. Like many filters, however, it also removes a significant amount of "signal" or good contributions that don't fit through the filter. Other media like UseNet have "moderated" newsgroups, where one or more people do the filtering. (Usenet also has many non-moderated newsgroups where the user's client software can do the filtering through means like "kill files".)
One difficulty of measuring online S/N ratios is that different people have different definitions of "noise". In the classical physical sense noise is a deviation from the expected signal. Online, people often have very different expectations about the signal. Many forums have offered optional "sections" or "areas" to separate content to help people filter out what they consider "noise". For example, some people don't think anime (a form of animation popular in Japan with many fans worldwide) is an appropriate topic for the SlashDot site--they can simply unselect that section and they will not be bothered with new anime stories.
Different communities vary in their emphasis on filtering. Some sites (like most wikis) provide almost no filtering and are difficult to filter with client software. They rely on PeerReview, peer pressure, or influential leaders to convince people not to contribute noise. Others like open mailing lists or unmoderated newsgroups rely on client-side filtering (and sometimes personal influence). Still others provide various forms of filtering from administrative/editorial actions to democratic/rating-based filtering.
There is often a conflict between groups that believes that better filtering is the best way to approach S/N problems and other groups that believe that better contributors are the key element, and that people should be influenced to stop adding noise to the system. Even within the "filtering" group conflict often arises over how much filtering control/work should be delegated to the individual readers (individual subscriptions and filters) vs. making filtering decisions that the user can't override (like score-0 comments in KuroshinMojo).
On at least one list I was on, pedantic and rude behavior toward newbies was the normal way of normalizing behavior. Either the person came into shape and became a welcome member of the list or they left. Either choice was acceptable, and it took care of the SignalToNoiseRatio problems that started occuring during BigSeptember?, especially after TheSeptemberThatNeverEnded. -- DaveJacoby
Another aspect is that the communication platform may impose a level of noise automatically on every message, also known as message overhead. This is stuff like message headers like from/to/date/time/timezone/subject/mailer/priority and environmental cruft like popup menus for filtering, buttons/links to "ignore this poster", message ratings, icons, borders, boxes, and so on.
Wiki's generally have less environmental noise than other forums, since each page has very little automatic cruft. This doesn't excuse deliberately low-signal messages, anymore than the quietude in a library is an invitation to sing.