They are all great sites that help bring website visitors directly via clicks. Then there's the effect that the "social media signal" has on your general search engine rankings.
But the greatest SEO benefits come from social media when you use it to put your content in front of other bloggers and webmasters in your niche. By doing this you hugely increase the odds of garnering quality one way links to it.
All the sites mentioned above can deliver this effect. But I think the best one of the lot is Googe Plus. Sure, it's not anywhere near Facebook in size, reach and activity yet. But it's growing apace.
There are several SEO benefits that accrue when you're active on the site. Perhaps the most obvious is that getting those "plus ones" on your articles helps lift them in the SERPs. Sharing them on this social network obviously increases their number.
So it's a very good idea to join this site now, get active on it and and start to build your network. Seeing the direction it is taking -- it's already a sophisticated melding of social network and search engine -- I suspect that there will be many more such benefits to membership coming down the track. It's one site that I'm going to focus on and I urge anyone with a website or blog to do the same.
You can connect with me on Google Plus here.