I don't cross-post the entire post.
At some forums which I own or am an active member at, I might copy the first-second paragraph and include a link at the bottom to read more (of course, this is done in discussion form).
With social networking websites such as Twitter, the title and link gets copied (and maybe if I can enter more characters in the submission field, I'll add a few words), that's all.
I tend to post a link to Twitter straight away but on FB I tend to use a small part of the write up to attract traffic to me website. Over a period of time if my traffic hasnt increased too much, I will post another extract of the article on FB
You can post your blog content to advertising content marketing on multiple social media. Cross promotion help to make your content more visible and get real traffic on your blog to boost traffic and ranking.
It's not a good idea to post the same article on multiple sites. Google might most likely see it like duplicates. Which means that the first article you publish, probably won't be affected, but the second and third won't rank to well.