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2.
A communique intended to spread awareness of an issue, typically through mass media such as amateur radio or the Internet. Sometimes knocked by detractors for unapparent reasons - voting doesn't raise money for anything, but voting is still important. Self-Injury Awareness Day doesn't raise money for anything, but awareness is still important. The entire point of a signal boost is that if you can do something more significant to help, you do that as well as participating in the boost, but if you can't, you boost to help reach those who can.
Tamara says she's standing on an eighth-floor windowsill and I don't know where she lives. Signal boost 'til we find someone who does.
by Apricot Man February 03, 2013
 
1.
Posting to a community forum (mailing list, social networking site, discussion board) in hopes of getting more attention for an event or cause. This is not the primary or first announcement, but rather one of many auxiliary posts or cross-posts to communities with individuals who are likely to take interest.
*Signal boost: Race against Lupus on Saturday!*

If you haven't heard, we're planning a fund-raising race this weekend to support research on SLE. Become a sponsor! Web site: ...
by phyzome January 04, 2010
 
3.
Spreading the word, or 'raising awareness' of an issue without actually doing anything useful (like, say, donating money). Popularly employed on Tumblr by image-conscious bloggers who want to look like they care deeply about a cause or issue, while they try to avoid investing any time, money or effort into said cause.
A Tumblr slacktivist reads a blog post from another Tumblr user who doesn't have enough money to pay rent and is shamelessly groveling for money on the internet.

Tumblr blogger: "Reblogging this is as good as giving money, right? I'll just tag it 'signal boost' so everyone knows how much I care about raising awareness and helping others."
by We Fought a Zoo October 03, 2012