The best replacement for collage is montage.

The best alternatives to collage are rated as following:
★★★★★  montage
★★★  college
★  mosaic
  assemblage

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Speaking about collage, one probably shouldn't forget about instagram.

Other search terms related to collage and their ratings are:
★★★★★ instagram
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Google Trends

Scale is based on the average worldwide traffic of collage in all years. Click on graph for more.
collage
montage
college