Here are my tips but perhaps there is much more you could do and this is why I am writing this. First to share these tips but also to get more tips from people who struggled with something similar.
a) Sites for these purposes
You are not really sure who is the visitor (skeptic in me tells that other desperate researchers) but why not for the start. Maybe there are more of these.
Both seem reliable, actually I was able to get few people through them. The problem is that you question yourself why would you participant go through such a site?
b) Social networks
Sure but how? You can start your own group or page but not one gives a damn about it. Luckily there are people who did it already, so post it there.
If those links do not work you will get there by searching "Research participants", "Participants", "Participants and researchers" and such. I have also found out you could use Facebook groups where you sort of "Advertise everything", so you can look for this too.
The issue may be similar as above. In addition to that I have contacted personally all my friends to share my study, created a Facebook page and invited other people to join it.
Honestly, I just had very bad luck with it or it does not work. I was doing a research once in a name of very serious and top academic institution but no one bothered to respond. So I am clueless about how other people are able to use this site for research.
If you do not have an account, it is too late in my opinion. It is all about number of followers and I think my only luck in this case was that I had a friend with about 400 of them who was happy to share it.
c) Community based sites
Then there are those site such as Craiglist or Reddit (more?). You are looking for your participants online, why not post it there then? I have tried both and saw other people adding their research there. The turnout is questionable in my opinion and it does not work that well.
d) Your local university network.
Contact course coordinators, lecturers or other people maintaining large classes, usually you want people outside your own field. It actually seems that lot of these people are willing to help you out if you write them a good letter to share. I've contacted three major institutions in my country and asked their Erasmus coordinators to mail my study to their students. I think people with a known ac.uk email can actually have more luck with this approach.
e) Corporations, NGO, Governmental organizations
Imagine you could get your hands on one of the following: https://en.wikipedia.org/wiki/List_of_largest_private_companies_in_the_United_Kingdom. The idea I've found on this page:http://www.statisticsconsultant.com/dissertation-advice/recruiting-study-participants/ (if nothing else, there is nice sample letter you can circulate) suggest contacting national companies to convince them because they control large sample pools. In my opinion you will be denied. My approach was more low level. I've tried government organization and non-government organization. The success rate is low, they are overflowed with similar request but I got some responses there too.
I have already mentioned that in relation to FB. I think lot of people really tried to genuinely help me, especially if you make it easy for them to share it (write the letters, provide links and such.) Problem might be that you probably do not want to ask too much of them.
This is all I got but I would be grateful to hear your thoughts on this. Maybe you've got some out of box tips. Maybe you had a success with particular platform.