There are many situations where job vacancies will need posting abroad. However, to make sure that your investment pays off, it is essential to identify the right market(s), to find out which sites work best there (for the type of profile that you are looking for) and, if you can, gain a basic understanding of the local culture. 

The following ideas will provide a base for you to start from and although we focus on Continental Europe here, the message extends to posting worldwide.

Choosing your markets

1. Narrowing the choice to your needs

Bar some cultural differences, the EU enjoys a good level of labour mobility.  However, choosing the right market(s) will depend upon the profile(s) that you are seeking.  Your main considerations will be the language(s) required, the location(s), the salary and the skill sets needed.

Language - consider which markets will provide the lingustic skills you require.  For example, with English being so widely spoken, a vacancy in the UK could well be filled by advertising in any number of markets where the skill sets match.  However for a vacancy in Germany you could also consider posting in other German-speaking markets such as Switzerland and Austria and even in markets where a good level of German is found such as the Netherlands, Czech Republic and Poland.
Locations close to borders often open up the possibility of advertising in neighbouring markets.  For example, posting in France or Germany for a vacancy just over the border in Switzerland is likely to bring in a very good number of candidates, all motivated by salary. 
Salaries have to be well thought through.  For example with the current employment and social advantages in Scandinavia, attracting a worker out from this market will generally mean aiming high.
Finally, and perhaps the main driver, are the skills required.  The UK has a long list of skill shortages.  Other markets have similar lists.  The key is identifying where surpluses exist and only research or experience will provide you specific answers.  One example is the potential pools of IT labour developing in markets such as Bulgaria and Romania that could meet shortages in Western Europe.

2. Narrowing the choice – market drivers

Having narrowed the markets based upon your needs, the next consideration is how to advertise.  Across continental Europe, the on-line recruitment market varies enormously.  Many factors come into play; the most flagrant (and measurable) is perhaps internet usage.  A closer look at the full EU data identifies a general East/West divide.  Generally the latter has higher penetration rates and in similar fashion, a North/South divide can be identified too, with the Scandinavian markets enjoying generally higher rates than their southern counterparts.

Another factor is the dynamism of the national job board markets.  The UK leads the European field with estimates ranging from 1k to 4k job boards.  With 13 million more internet users, Germany has far fewer job boards (with estimates ranging from 100 to 1000) and the Netherlands, which enjoys a very high internet/population rate, has even fewer job boards (with estimates quoting less than 50).  Eastern Europe, the Baltic states and, to an extent, the Latin markets have notably fewer job boards than their western/northern neighbours.

Finally, your market choice might be affected by how you can accomodate relocation costs and visa requirements.