How Much of Facebook Traffic Still Bypasses Browser Cache? New Findings

Eight years after Yahoo's extensive research on web caching, Facebook's web team conducted a new study to measure current cache behavior on both desktop and mobile platforms.

Methodology

The team created a PHP endpoint that serves an image with specific HTTP headers and logs each request in a database, allowing them to track cache hits and misses.

Cache-Control: no-cache, private, max-age=0 ETag: abcde Expires: Thu, 15 Apr 2014 20:00:00 GMT Pragma: private Last-Modified: $now // RFC1123 format

Due to known bugs in IE7/IE8, the headers were adjusted to:

Cache-Control: private, max-age=0 Pragma: no-cache

Request Flow

If the browser has never requested the image, the server returns 200 Success with the image data, and the browser caches the Last-Modified and ETag values.

On subsequent requests, the browser sends If-None-Match or If-Modified-Since . If the resource is unchanged, the server returns 304 Not Modified , avoiding re‑download.

Data Collection

The image was embedded in Facebook’s search bar so that each page load triggered a request, allowing the team to collect weeks of data.

Results

Analysis of a week’s worth of data showed that 25.5% of requests were from an empty cache. Breaking it down, desktop browsers had a 24.8% empty‑cache rate and mobile browsers 26.9%.

Chrome and Opera exhibited higher cache hit rates, while Firefox data was excluded due to inconsistencies.

Overall, 44.6% of users experienced an empty cache, closely matching Yahoo’s 2007 findings.

Further analysis indicated that on mobile, 50% of requests had a cached response after about 12 hours, and 75% after roughly 20 hours.

These findings suggest that while cache hit rates have improved slightly since 2007, a significant portion of traffic still bypasses the cache, and cached resources tend to expire relatively quickly.

Practical Recommendations

To maximize cache efficiency, the team advises using external CSS/JS files, setting proper Cache-Control and ETag headers, compressing assets, employing versioned URLs, and separating frequently updated resources.

Further Reading

Steve Sounders’ YUI study follow‑up

Facebook code push schedule

Chromium cache metrics

IE9 Cache Enhancements