How does semantic search, social signals and Google rankings are related to each other?

Social signals a weak factor is incredibly wrong according to numerous sources. Aside from the fact that as you rightly point out they bring actual visits (and a lot of them too) they were in fact some of the most important ones up until this year. They still are except a few more metrics are getting into the top of the list.

The way social signals and semantic search are affecting the rankings is the concept of branding.

Google is interested in you creating your own brand and having people check for your brand name on the internet. This brand recognition is starting to be looked upon by Google as a sign people recognise you and actively search for what you have to offer (indicating an implicit interest in the content you provide).

Another thing semantics related, albeit on a side note, that Google is starting to rate is your SERPS CTR.

They have metrics of standard CTR for each position on the SERPS and when your CTR gets above what they think you should have for the position you are in they test you for a higher slot. Te bad thing about this is that the opposite is also true; if your CTR is lower than it should you will be "split tested" with a candidate for your slot, taking you down. Mathew Woodward has some info regarding the mechanics of this stuff if you want to do further research.

The way to improve your CTR? Higher visibility and copyriting. Write descriptions that will entice people to click on the link to see the full message and increas visibility with things like rich snippets ( i think the author pic form g+ is now gone, that was a good one too).

Of course all of this applies when you are being tested alongside a nother result whose aother algorithm metric are similar to yours. If you have good CTR and then user experience is click, stay 2 seconds, click back button you won't compete with someone that retains a customer with great content and has them click a second lnk inside the site to get more (the old cracked.com with their split lists were good at this). There have been a few theories about how Google can actually track user experience on sites that seemingly don't have any relation ship with Google. John Linbocker claims to have found the answer do a search for Linbocker safe browsing and enjoy the long video).

Of course if you get FCS Networker, Scrapebox, GSA SER and a few proxies you can start creating tiers like there is no tomorrow and that will get you far, at least for a while.

Or you can read that Oracle "repurposing content" pdf doing the rounds all over the place and mix and match you different hats for the task

Hope this helps.

If what you mean is that on its own it is not a big enough factor to I might argue with it.

If you really think it has no impact whatsoever in any way or form I strongly recommend you run a few tests, specially with the incredibly biased tendency of G of favouring sites with a lot of G+ likes.

Social does have an mpact on serps, it's just not as linear as it has historically been page rank or number of links or other factors.

Oh, and the weight each factor has changes year on year too. Who would have thought. Here I was thinking a blackhat linkblast would put me on top spot for weight loss, jut like n the 90's. Silly me...