How is what GIPHY is doing not just plain copyright infringement? I get the "search Engine" spiel, though questionable, on their site. But there is no attribution on Twitter/Slack etc.
Fair use is hard to determine before you get sued. There is no black or white rule. There are 4 (or more) factors to consider. Some are in favor of it being fair use--a)Amount and substantiality* and b) effect upon work's value. Some go against fair use, like commercial nature of the work. The fact that Giphy is just ripping off other people to get funding is bad for their fair use argument.
How a court would rule isn't clear.
*assuming they are copying a clip from a movie or show. Ripping off someone's original gif would turn this factor against them since they copy the whole thing.