too many link posts have any bad impact on site ?
Not really. The only real issue this may have is potentially linking to bad neighborhoods but if the content is moderated in some way that shouldn't be an issue.
How google or any other search engine measures the value of such posts?
The same as they do any content on the web. They use semantic markup, anchor text, etc to determine the page's relevance for any given search term.
Also how value of link is passed ? or all values to links in the posts are passed equally ?
The same as any link on any other page. The more links on the page, the less PageRank is passed to receiving pages. The less relevant the page is for any given search term the less value it offers pages that are trying to rank well for that search term. etc.
From Matt Cutt's blog post about this topic:
I’m about to publish a blog post with a ton of links in it — almost
two hundred of them. So before I did that, it seemed like a good time
to talk about Google’s recommendation to “Keep the links on a given
page to a reasonable number (fewer than 100).” Why do we provide that
recommendation, and what if you decide to ignore that guidance?
The original reason we provided that recommendation is that Google
used to index only about 100 kilobytes of a page. When we thought
about how many links a page might reasonably have and still be under
100K, it seemed about right to recommend 100 links or so. If a page
started to have more than that many links, there was a chance that the
page would be so long that Google would truncate the page and wouldn’t
index the entire page.
These days, Google will index more than 100K of a page, but there’s
still a good reason to recommend keeping to under a hundred links or
so: the user experience. If you’re showing well over 100 links per
page, you could be overwhelming your users and giving them a bad
experience. A page might look good to you until you put on your “user
hat” and see what it looks like to a new visitor.
But in some cases, it might make sense to have more than a hundred
links. Does Google automatically consider a page spam if your page has
over 100 links? No, not at all. The “100 links” recommendation is in
the “Design and content” guidelines section, and it’s the Quality
guidelines that contain the things that we consider webspam (stuff
like hidden text, doorway pages, installing malware, etc.). Can pages
with over 100 links be spammy? Sure, especially if those links are
hidden or keyword-stuffed. But pages with lots of links are not
automatically considered spammy by Google.
So how might Google treat pages with well over a hundred links? If you
end up with hundreds of links on a page, Google might choose not to
follow or to index all those links. At any rate, you’re dividing the
PageRank of that page between hundreds of links, so each link is only
going to pass along a minuscule amount of PageRank anyway. Users often
dislike link-heavy pages too, so before you go overboard putting a ton
of links on a page, ask yourself what the purpose of the page is and
whether it works well for the user experience.