Did you find your sentences here? http://www.ielts-writing.info/index.php/academic/ielts-writing-task-1-13/ The example sentences are correct, however you have taken the first example out of context.

As Ron Jeremy wanted to judge you instead of answering, i'll have a go. 1. In 2007, the number of home owners had risen to 70%. (Since we are dealing with percentage here, should we use "percentage " or "proportion" instead of "number"? ) Depends what the context is, but when dealing with mathematical terms it's best to use whatever the symbol denotes, which in this case is 'percentage', but if you wanted to reword it you could use 'proportion'. Eg 'The number of home owners had risen to a proportion of the population equal to....' etc. 2. Social Housing has decreased three-fold from 6% in 1991 to 2% in 2007. (Should I add "The proportion of" before "social housing"? ) You could do if you wanted i think but there is no need to.