Remember that social media is a place where people communicate
Search out the profiles of people with blogs
Look at blog lists to find social media widgets and see where they lead
On other peoples blogs you are going to find attempts to link social media and their blog. It may be done with links to their social media profiles, but it most often done with widgets of different varieties. Follow these widgets and you will find their social media profiles. On their social media profiles you may start to engage with them a little. You can talk about their blog and stroke their ego a little. Once you have buttered them up a little, you can start to suggest over social media that maybe you could guest post for them.
The blog search tool is the best way of doing this sort of thing because you can search for blogs that are similar to your own. Getting links from websites that have the same target audience as you is a good idea because it makes your blog more search engine friendly. It even makes their blog a little more search engine friendly because Google takes outgoing links into consideration these days.
Comment and engage with people on their social media profiles
Ask other people where the best guest posts are
You can do the same on Facebook and Google+. You can even try a few bookmaking sites to see if any good blogs have been bookmarked that also allow guest posters to upload. If you play around on social media for long enough you are bound to come across some useful information without having to try too hard.