While its true that Neilsen doesn't track each home, but it does use standard sampling patterns (those are usually accurate within a small margin or error). As for online viewing (at least through legitimate sources) that is tracked to a very accurate degree. They get very accurate data points of of online viewing.
The main problem with online viewing is that up to this point the number of ads per hour show is considerable smaller then what occurs on tv. And unlike ads for TV (where they have breakdowns of the income level, sex, and age groups that watch their programs and can sell ads that target those groups, those don't occur with online viewing and as such advertising revenue off of online sources is still very marginal.
This upcoming year though, two major things or occurring as far as online viewing. 1. Many sites are now going to be airing he same commercials that air on tv (so instead of a more common online ad period of 4-8 minutes you will see ads increasing to 20 plus per hour show). That will increase revenue to be closer to one online viewer being worth one live tv viewer (it will still be less, but much, much closer).
Also Neilsen is starting a target program to start including those viewers as part of their ratings that will help set ad rates (it's a pilot program so this year will really be a test). Currently ads for internet are not sold in the upfront packages. A year or two down the line this will change.
Also the time a show airs also heavily impacts on when it is watched off of DVR's. Shows that air at 10 typically have most of their DVR viewing the next day or later (and have the largest percentage that is after 3 days), shows at 8pm have the highest rate of DVR viewing during that same day, and the lowest percentage of viewing outside the 3 day.
As for DVR use while time shifting does occur, revenue is only generated during the first 3 days of airing. After that if you watch a show (even if you are a Neilsen family) you really wont be helping your show at all.
For TAR (and in fact almost all reality shows) the largest part of their time shifting is the same day that the show airs. Thus maximizing any potential ad dollars.