According to the recent report from Gartner, Apple's tablet has lost more shares last year. The report clearly shows a marginal increase units per sale in the year 2013 from 61,465,632 to 70,400,159 but the company has now dropped its market share from 2012's 52.8% to 36.0% (2013).
On the other hand, Android tablets have appeared to be on the top ranking when it comes to progressing. It has grown massively both in terms of sales per unit and market share. Compared to the year 2012 which saw the units per sale at 53,341,250, in 2013 it increased to 120,961,445 which is more than expected. Also the market share has taken a huge leap from 2012's 45.8% to 61.9% in 2013.
The Windows tablet still remains in the third position with 4,031,802 units per sale in 2013 from 1,162,435 in 2012. But the share is comparatively very less compared to iOS and Android tablets.
Overall the tablets shipment has increased from 116,348,317 in 2012 to 195,435,004 in 2013. But vendor-wise Apple still remains at the top position with 70,400,159 sales per unit. South Korean Samsung comes second and Asus follows on third.
Also the report says that the sales of tablets was boosted by the low-end tablets and hence Android continues to dominate the tablet market. There are several Android tablet makers which have been offering their product at a very marginal price. Also Samsung's mid-range tablet can be credited in most of the countries like India and other Asian markets.
You can have a look at the figures above.