But the domains can be divided into general classes of users. If jawbone can recruit me for it UP band then withings can also recruit me for its body analyzer. Not all startups in health segment have do its own health conscious user recruiting.
There is a lot of overlap between products and I am sure there is some market for a generic user recruitment.
My suspicion is that you don't want generic users. You want users who are going to stick with your product for a while and not jump from one train to another. Without them thinking that your product stands out changes are pretty low that they are going to talk with somebody else about it.
Upon seeing those words, I realized that's what a lot of media startups are. For example, my companies runs a variety of e-mail newsletters and most of our advertisers are other companies looking to acquire users/customers. We've had advertisers run a single ad with us and pick up 1000 e-mail addresses on their landing pages, etc. That seems quite close to a practical user-acquisition startup even if the intent isn't there.. It does give me some ideas on extra ways to sell the value to potential advertisers though(!) :)
Essentially, every startup is a user-acquisition startup in its own domain, and it's hard to beat the good ones at their specialty.