Both Twitter and Facebook are excellent because you can easily target the people you want to target. If Google had a good social network (not Google Knol!!) then I would put them toward the top.
good social networking need time for setting complate profile. you have to spend lot of time for creating related groups.
facebook is better if you want to find your friends from school. because many people have account in face. but twitter seems much mıore formal i can say for corporations.
Twitter is probably better for advertising because your post can be an ad. With facebook i rarely, if ever check out the ads and if i do i don't pay them much attention because most of them dont seem useful.
Facebook is great for advertising. Their prices are affordable and yes, it's hard for spammers to succeed there, that's why I love it lol
facebook is best social network , and its very easy to advertise there. You can create groups , can create fan pages and lot more which can make attention to your site and thus advertise your product
These days popular and powerful social networking sites are... - live journal - blogspot - wordpress -Twitter - ezine -Article101 -facebook -my space -multiply -Orkut Hope it helps you.