TMW, there is a very social aspect to most games now. Its just like t'internet, you communciate with people in different locations, so you may be physically alone - but you're still communicating and being sociable.
TD, it depends what age your children are whether the Wii is the best bet. We've had Wii, XBox and PS3 in our house and the Wii is the one that quickly gathered dust as the majority of games available are rubbish and the good ones get a bit repetitive after a while - but our 6 year old still sometimes wants to play it, however our 11 year old has "outgrown" it. I haven't seen anything about the Wii U that is going to change that.
Xbox 360 and PS3 are much the same (new versions will probably be 12 months time for us after initial Xmas offering in USA & Japan). The PS3 is a more complete package - it includes a Blu-Ray player and also the online stuff is free (whereas Xbox requires XBox Live Gold membership @ circa £35 pa for full online feature). Other than that there's very little to differentiate them for the casual gamer, as the major games are released on both platforms.