I just found out that google doesn't give any weight to sitewide links anymore. I have done about 50 sitewide link exchanges with other related sites. I kept very high positions for about 2 months for very competitive keywords, and my site dropped since the last PR update. Any thoughts?
Could it be you are confusing poor Google with 50 sitewide links to unrelated sites? Could the PR drop be because you are sharing your PR with 50 sites ... err Sitewide?
This is not always so clear what sitewide is and what is not, but as I noticed, sitewide doesn't give much. Surely 50 backlinks on single site, as sitewide, has not much power as 50 backlinks from various different sites.
Many identical links from different pages on a domain to another site? Sites built upon templates often contain identical links back (phpld as an example). But then that's another reason not to use templates. In general, it'll cause the source site to loose PR and unless the content on either side of the links relates, it'll have limited benefit to the recipient. I'd suggest (as a rule of thumb) links from less that 10% of your site's pages to the destination domain. Otherwise it would appear to be a form of link/PR directing.
every link will have your serp. if you have site wide links on a site, first link will have a big weight but the second and the other links will have lower weight for your serp but every link will help you with different weights