My site has been up for 3 months now. it is fully indexed by google and msn but not by yahoo!. My pages are optimized and I have over 100 backlinks. Is it just harder to get fully indexed by yahoo? does it take longer than the other search engines?
It takes much longer. Yahoo directory helps, but you'll still need a lot more than 100 links (or some really good ones) to get indexed quickly and regularly.
How many links do you think will be needed to be full indexed by yahoo!? My site has been live for 3 months now
Depends on the quality of the links, and the time period. Make sure the page you get links from has a lot of inbounds - Use Yahoo to check, of course. In general, it takes a while for Yahoo to fully index brand new sites. It's almost like a mini-sandbox. But once your site has some age, and more IBLs, Yahoo's SLURP will do a pretty good job. Don't rely wholy on pagerank when looking for links, rely on # and quality of inbounds they have (and relevance, of course).