diff --git a/silobridge/silobridge.go b/silobridge/silobridge.go
new file mode 100644
index 0000000..1cc0d2d
--- /dev/null
+++ b/silobridge/silobridge.go
@@ -0,0 +1,29 @@
+package silobridge
+
+// fakeAPInstances contains a list of sites that we allow writers to mention
+// with the @handle@instance.tld syntax, plus the corresponding prefix to
+// insert between `https://instance.tld/` and `handle` (e.g.
+// https://medium.com/@handle)
+var fakeAPInstances = map[string]string{
+	"deviantart.com": "",
+	"facebook.com":   "",
+	"flickr.com":     "photos/",
+	"github.com":     "",
+	"instagram.com":  "",
+	"medium.com":     "@",
+	"reddit.com":     "user/",
+	"twitter.com":    "",
+	"wattpad.com":    "user/",
+	"youtube.com":    "user/",
+}
+
+// Profile returns the full profile URL for a fake ActivityPub instance, based
+// on the given handle and domain. If the domain isn't recognized, an empty
+// string is returned.
+func Profile(handle, domain string) string {
+	prefix, ok := fakeAPInstances[domain]
+	if !ok {
+		return ""
+	}
+	return "https://" + domain + "/" + prefix + handle
+}