Did you try http://www.clickworker.com/. (formerly known as "humangrid")?

I guess they were the first in Europe - I think even before Amazon Turk entered the market

Research Now, a sub group of eRewards, Inc., is a global, online survey company which people sign up to be a member, eRewards then emails them surveys, and rewards the completion of surveys through points which can be cashed in for different things such as gift cards. A Ph.D. student I am working with recently used this company for a study in the area of social media and had very good success. This information is from her dissertation.

Several studies regarding SNS have utilized this service in their data collection strategy. For example: Uhrig, Bann, Evans and Williams (2010) used eRewards for their study entitled Social Networking Websites as a Platform for Disseminating Social Marketing Interventions: An Exploratory Pilot Study;

Horvath, Courtenay-Quirk, Harwood, Fisher, Kachur, McFarlane, & Rosser, (2009) utilized eRewards in their study Using the Internet to Provide Care for Persons Living with HIV.

Here is their own statement: e-Rewards®, Inc. (ERI) is the global leader in permission-based digital data collection and reporting. From its inception in 1999, e-Rewards, Inc. has become recognized in the market research industry for setting high quality standards through the combination of innovative technology and proven research practices – all toward helping clients discover insights that lead to greater understanding. With over 1,200 employees worldwide and 6 million panelists around the globe, that ‘world of understanding’ is becoming more and more attainable (e-rewards, inc.com, 2014, n.p.). 

Hi Mona,

Co-founder of Prolific Academic here. We've developed an alternative to MTurk tailored for academic research and we are accessible worldwide. Over 200+ researchers have completed over 750+ studies on our platform so far.  

New users can run a trial study for free: https://www.prolific.ac/rr?ref=5ZFZ276D. You can find more information here: www.prolific.ac

Should you have any questions about the platform please don’t hesitate to get in touch.

Cheers,

Katia

www.prolific.ac

Hi Daniel,

Prolific works such that you insert a survey link in order to start collecting responses (you can find more info here: https://prolificac.zendesk.com/hc/en-us/articles/202820772-How-do-I-post-a-study-on-Prolific-Academic-). We do not provide representative samples as such, but rather randomly chosen sub-samples of all participants that are eligible your study. I'm not entirely sure about the other platforms. Hope this helps :)

Any alternative to prolific that allows segmenting the respondents with our own criteria? None of them (mturk, prolific, ecc) allows this filtering question!!

www.prolific.ac this looks helpful! I will sign up and reach out in case have any questions. Need to gather primary data. Thank you for sharing this :-)