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Cloud.li is a cloud-based Twitter search engine that provides you with a list of commonly used keywords and phrases for another phrase, in cloud form. Type in "Mashable" and you'll get a cloud of keywords, those most prominent being the biggest in the cloud; in this case, those are "social, media, facebook, behavior, user, change, etc." Now, click on any of those words and you'll get a Twitter search for the chosen word plus the keyword "Mashable." Simple and quite useful. You can also exclude certain keywords to narrow your search down.
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