Someone tell me please: if I link to other sites, I give some PR to them, but will it decrease the value of the pagerank of my page I link from? I hope it is not harmful to put links to other trustworthy websites? What is the optimal number of external links from my page I can create? Or maybe I should avoid any external linking at all?
linking to sites that you feel are "trust worthy" or "quality" should never hurt you, as that is how linking should work. As for how many are are too many, look at any given page you have out going links on and ask yourself "how does this look?" and if you think it's fine, it should be fine... however, if you think "hey, this looks a little spammy" then chances are Google will as well. All in all, if we all started to share a couple of links to quality sites freely, the search results would be less spammy and we would all have a happier web experience. The best outgoing link is one that is natural. In content is better for both sites, footer links are ignored by humans and likely discounted by bots, link lists are also generally discounted by many... but again, just use your best judgment, in both the number of links, and the quality of the sites you are linking too and you should be OK.
Many people buy footer links on templates, so I'd assume they're worth something to search engines. Have you ever seen those footers that say something like: "website designed by 'online payday loan' "
as a general rul linking out leaks PR, which means it reduces your pr. but if you would link to authority sites in related niche, that shouldnt be a problem.
linking to other sites of the same category actually HELPS alot via seo. Google has the whole 'Share the Wealth' idea when it looks for that.
yeah, and I have bought some... have I ever clicked a footer link? Nope...I generally do not scroll down that far. Do bots care? Chances are NO... Google does its best to make it as natural as possible...as people do not naturally click footer links, then naturally Google will discount them. Does that mean people are not buying them? Well see this thread: Do DP members really know how to build links?