Some preliminary results of the recent survey on use of machine translation have just been published on translation service home page of the French Linguistics site.
As you will see, there is actually quite an even split among the different reported frequencies, with around the same percentage of users reporting that they use machine translation at least weekly (21%) as those reporting that they never use it (26%).
Note that the category "Once or twice" (18%) refers to those users who in the survey reported that they had previously used machine translation "once or twice" but did not report to be regular users. The category of "now and again" (18%) refers to those users that report regularly using machine translation every now and again.
Given its ubiquity on the web, it is interesting to see that almost a quarter of users report never having used such a service.