Booking through cataloguing

Do you use any of the online book-cataloguing sites, like Library Thing or Shelfari? Why or why not? If not an online catalog, do you use any other method to catalog your book collection? Excel spreadsheets, index cards, a notebook, anything?

I have seen way too many books to use online cataloguing sites. I am a member of both Library Thing and Shelfari. Somehow I do not use those.

Right from my school days, I used the indexing method. I sorted out my books and numbered those accordingly. Genre/Author/number. It was my own personal library. Now too, when I get time I kind of put numbers on my books. I am way behind but I can still know where any of my books is placed. I can pick it from that place in a sec. It is kind of mental cataloguing. I do maintain a notebook, for I don't remember how long. I am trying to convert into Excel spreadsheets. Not an easy feat, considering I have 4000+ books. I need to drastically reduce the number. I just have not got around it. I buy more than I dispose off.