If you’ve ever analyzed traffic from your Facebook ads, you’ve likely come across shim links like lm.facebook.com and l.facebook.com. Shim links may seem unimportant, but they play a significant role behind the scenes in digital marketing.
In this article, we’ll explore what shim links are, how they connect to Facebook’s approach to security and privacy, and why they matter for your analytics.

What Are Shim Links?
Shim links are re-written URLs that redirect traffic through an intermediary link before reaching the final destination. This technical process is vital for enhancing security and complying with privacy regulations, but has an impact on data analytics and how marketers should assess traffic sources.
Security
One of the primary functions of shim links is to enhance security. Before redirecting users to their intended destination, the intermediary link performs a quick security check to ensure the destination is safe. This process, which happens in just milliseconds, helps identify and block potentially harmful sites such as those involved in phishing, malware distribution, or other malicious activities. If the redirecting site (such as Facebook) flags the destination as suspicious, it can alert users and prevent them from proceeding. This added layer of protection helps safeguard users from online threats.
Privacy
Shim links also serve an important role in protecting user privacy. Referral URLs can often contain sensitive information like user ID, tracking parameters, or session data. Shim links anonymize this information, ensuring that it is not passed along to third parties when users click a link. This anonymization process prevents personal data from being exposed and provides an extra layer of privacy for users, which is especially important in an era of heightened concerns about data sharing and security breaches. This practice also helps platforms like Facebook comply with strict privacy laws such as GDPR and CCPA by safeguarding user data and limiting the risk of unauthorized data sharing.
Analytics
Shim links are valuable tools for marketers and analysts. Without them, traffic from certain sources might show up as "unknown" or "direct" in analytics tools, making it harder to identify the true source of a user. Shim links help solve this issue by identifying the referring site, such as Facebook, in tools like Google Analytics. This level of detail allows marketers to better understand high-level traffic patterns and attribution based on referring URLs.
Impact on Marketing
While shim links improve referral accuracy, they can introduce a layer of complexity when it comes to understanding the granular source of traffic. Seeing a referring URL like l.facebook.com clearly indicates the traffic originated from Facebook, but this broad classification doesn't offer any insight into whether the lead came from a paid Facebook ad, an organic post, or even a shared link within a private group. This lack of specificity can be particularly limiting when you're trying to assess the performance of specific marketing efforts (such as measuring the ROI of a newly launched paid campaign).
One way to gain some clarity is by monitoring the volume of shim link traffic before and after launching a new campaign. A noticeable uptick in shim link traffic following the launch of paid ads would suggest that some of this traffic can be attributed to the paid efforts, even if you can't directly identify which ad generated the clicks. In this scenario, analyzing trends in shim link traffic can help you estimate how much paid traffic is being obscured by shim links.
Marketers can incorporate that estimated "lift factor" into their efficiency calculations to account for the masking effect of shim links. By factoring in this potential increase in traffic that can't be directly attributed to a specific campaign, you can prevent paid campaigns from being undervalued during analysis. This strategy helps create a more accurate picture of campaign performance, even when shim links are involved, allowing for more informed decision-making in future marketing efforts.
Shim links are small but powerful tools that highlight the intersection of security, privacy, and actionable insights in digital marketing. By enhancing privacy and security, they protect user data and shield against online threats. For marketers, shim links can complicate tracking - improving referral accuracy, while obscuring some more granular sourcing data. Understanding their role allows businesses to monitor the impact of shim links on their data and make decisions about how to best account for them.