I was curious if I could measure IFTTT traffic to my site so I setup a simple RSS-to-email recipe while tailing my access logs. Turns out their user agent string only identifies itself as 'curl':
54.172.140.57 - - [11/Oct/2016:22:00:59 -0700] "GET /feeds/atom.xml HTTP/1.1" 200 13473 "-" "curl" "-"
52.91.39.135 - - [11/Oct/2016:22:06:09 -0700] "GET /feeds/atom.xml HTTP/1.1" 200 13473 "-" "curl" "-"
54.162.188.32 - - [11/Oct/2016:22:11:15 -0700] "GET /feeds/atom.xml HTTP/1.1" 200 13473 "-" "curl" "-"
54.204.197.181 - - [11/Oct/2016:22:16:16 -0700] "GET /feeds/atom.xml HTTP/1.1" 200 13473 "-" "curl" "-"
107.22.155.34 - - [11/Oct/2016:22:21:21 -0700] "GET /feeds/atom.xml HTTP/1.1" 200 13473 "-" "curl" "-"
54.84.202.24 - - [11/Oct/2016:22:26:47 -0700] "GET /feeds/atom.xml HTTP/1.1" 200 13473 "-" "curl" "-"
54.165.18.27 - - [11/Oct/2016:22:32:33 -0700] "GET /feeds/atom.xml HTTP/1.1" 200 13473 "-" "curl" "-"
54.237.233.244 - - [11/Oct/2016:22:40:11 -0700] "GET /feeds/atom.xml HTTP/1.1" 200 13473 "-" "curl" "-"
54.211.88.14 - - [11/Oct/2016:22:46:07 -0700] "GET /feeds/atom.xml HTTP/1.1" 200 13473 "-" "curl" "-"
54.235.54.123 - - [11/Oct/2016:22:52:15 -0700] "GET /feeds/atom.xml HTTP/1.1" 200 13473 "-" "curl" "-"
54.198.2.88 - - [11/Oct/2016:22:58:35 -0700] "GET /feeds/atom.xml HTTP/1.1" 200 13473 "-" "curl" "-"
184.72.126.209 - - [11/Oct/2016:23:06:11 -0700] "GET /feeds/atom.xml HTTP/1.1" 200 13473 "-" "curl" "-"
Additionally IFTTT appears to check the site for updates every 5 minutes, and all the IPs I ran though IP geolocation were from the us-east-1 AWS region. This tweet suggests the same, and that there are a lot more IPs than I saw. Unfortunately their engineering blog isn't very active anymore but they do have a fantastic post about their infrastructure.
Running curl from macOS and CentOS included a version number in the user agent:
[11/Oct/2016:22:08:15 -0700] "GET / HTTP/1.1" 200 9458 "-" "curl/7.49.1" "-"
[11/Oct/2016:22:08:36 -0700] "GET / HTTP/1.1" 200 9458 "-" "curl/7.29.0" "-"
In fact running zgrep "\"curl\"" * to scan my gzipped access logs for that naked curl UA among all the sites I administer only turned up IFTTT hits. Maybe behind the scenes IFTTT really is just making curl calls, and hiding the version token is simply to deny a piece of information about their systems to potential attackers? A more cynical mind might think they were trying to hide their traffic so it would be harder to identify or block.
Whatever the case, monitoring for a version stripped 'curl' UA appears to be the best method to guesstimate your IFTTT traffic currently, especially since the requests themselves come from many different IPs which could easily change over time.