You mentioned the difficulty of evaluation, and I think this is a major point to be approached.
However, when practitioners of participation (both online and offline) think of evaluation there is a current assumption of conceiving evaluation in quantitative terms (e.g. numbers of people participating) and / or in qualitative (e.g. the quality of deliberation) terms. Even though such dimensions are essential to a good evaluation process, they are only taking into consideration those who participate.
There seems to be an implicit assumption that, for instance, if not many people have participated in a consultation, such a consultation was a failure. I believe that in many cases this is an erroneous assumption. A step further should be taken when considering evaluating participation initiatives. Approaching participation more as an opportunity and as a “right”, takes participation initiatives to a perspective that encompasses not only those who participate, but also those who do NOT participate. And; in this sense, it is essential to understand the attitudes of those who do not participate towards participation initiatives.
I explain myself. The fact that I have freedom of speech does not imply necessarily that I am making use of this freedom permanently. Most of my daily talks could take place in Cuba; China or North Korea without any problems. However; the fact that I rarely make effective use my freedom of speech does not mean that I am indifferent to such a right. Actually it is one of the rights that I judge to be most important ones.
In short, despite the fact that citizens do not take advantage of all opportunities and rights that are offered to them, this does not imply that they are indifferent to them. Maybe citizens are not participating just because they are satisfied with the services; but these same citizens are extremely happy with the existence of such a channel of participation that they may use it whenever they judge it necessary.
In this sense; an evaluation should take into account the non-participants. For instance; a survey of non-participants could allow you to know that despite a relatively low level or participation, most of citizens judge such an initiative extremely important. Consequently; the implemented initiative is a success to a large public in terms of enhancing legitimacy and trust in governments. You can also find more practical information such as: 1) If participants know about the existence of the initiative 2) Why they are not participating 3) What would make them participate more 4) …
Briefly, when thinking of evaluation of participation initiatives, think of the non-participants as well, and you might come out with stunning findings.