In what context are you referring to use of the term "success"? For this forum, I suppose that it's just whatever someone who posts his or her success story considers to be a success.

Over on the section of the eH website titled "Success Stories," it looks like the couples featured there are all at least in serious, committed relationships ("in love"). The form for submitting one's story includes the following language: "If you're dating, engaged or married, please submit your information today, so we can keep in touch with you as your relationship progresses." I expect they don't post stories of the "We met 3 weeks ago and now we're in love!" nature, though...lol (you have to fill out the first page of the form to get to the next one, so I can't tell what sort of other information they might solicit about one's relationship before determining that it should be added...).

In any event, it appears that "success" is not measured only by marriage, or even being engaged. Perhaps it is that one has found love...

You are right, though, that there can be many different measures of "success" (successful match, successful relationship, successful marriage...). For eH, they at least want to produce successful matches, but it's clear that creating successful marriages is also a long-term goal for the company and I believe that some of their research has focused on that...