First they ignore you, then they laugh at you, then they fight you, then you win
I am a regular contributor on HN but I work in this industry and so i am using a throwaway account.Dan, 
1.3 billion 'visitors' is a really vague representation. Is it user sessions or page views or number of unique visitors measured in one of the experiment/control groups?Test & Target mbox hell as mentioned in the post is very very true. Learning to use it within one hour is a joke. The technology they use is really stale and hasn't changed a lot in the recent years. Many of the players who started after 2008/09 have far superior implementations, thanks to lot of advancements in browser technologies which T&T failed to capitalize.I am very happy for Optimizely's success and these two posts will bring them a good amount of well deserved PR. Test & target and in fact a good number of other providers are way ahead of Optimizely in this game. The number of sessions/experiments is one good metric but it is definitely not the best one to measure the leader in the race.* What is optimziely's ARR?At 2800 customers paying $250/month [1], it should be around $8.5M. Lets be generous and make it $10M. A handful of T&T's high profile clients will be paying Adobe that amount.* How many IR100, IR500 [2], top 100 travel, publishing, finance sites are in Optimizely's client list?Builtwith's data is not very accurate. We have worked with the guy from builtwith to compute some lists and their methodologies are not very great. They just look for signals in the script tags and it throws lot of false positives. Out of my own interest, i have spent hours writing parsers and scripts to compile the list and Optimizely is not really in that picture.You can have thousands of clients paying you hundreds of dollars or have a few hundred clients paying 5/6 figure amounts. Optimizely has been after the former while the other players including T&T have been targeting the latter. I guess Optimizely is now trying to expand their enterprise client list since that is where the big money is but it takes a lot more to compete in that arena with long sales cycles and vast requirements.* How many user sessions are being served?Requests is probably the worst measure. T&T mentioned it because they can show their artificially inflated metric which is a result of their terrible implementation. IMO, user sessions is a better measure. 1.3 billion user sessions (in 2 years)[3] is still no where close to what the other providers serve. Number of tests currently running is also a good measure.Optimizely is a great product and they have made it super easy to run A/B tests but false proclamation claiming you are number one when you are not even close is not very nice.[1] Optimizely probably has a good number of clients in that 2800+ who pay $10k-20K a month. The highest openly priced plan is $360. I guess it wouldn't be wrong to assume that 50% of the customers will be in one of the two lower plans ($17 or $71). Optimizely's current ARR would be somewhere between $5M to $10M (My guess!)[2] http://www.internetretailer.com/top500/list/[3] There was no mention of the timeline. So i assume it is from when they started in 2010.