Seems to me that the 'most single' countries are also ones where women are unlikely to have the same level of internet access as men and where women's social position is considerably subservient to men's. There may also be an element of 'youth skew.' Less-single countries seem to have a higher technical sophistication in general, so there are likely more older not-single people online there.
In some countries, reporting yourself as single may be less common because the line between single and being in a relationship is somewhat blurry, at least for younger demographics, and people don't want to be faced with the awkward situation of having to decide when to change their relationship status. Easier just to leave it blank. Just pointing out it may have nothing to do with privacy concerns.
Numbers are probably skewed because people put false relationship data on facebook.
My feeling is that the numbers are skewed because of the bias in the people choosing to report their relationship status. For the countries with a low number of reported single people, perhaps there is some stigma attached to being single?
numbers may be skewed, true. but in aggregate truth will out. perhaps some people are not truthful in their reported data (any data anywhere), nevertheless all you need to do is make assumptions on the veracity of the data and what you get is a +/- range from your starting point. that said, these numbers are kind of what you would expect if you just had to guess or rank.