Remember that Google tries hard to give less weight to links that we've set up for ourselves.
Many social profile pages do get indexed but links from social profiles will have vanishingly small SEO value unless they have earned some honest link juice and visibility of their own. Even then there's no guarantee.
If all you do is set up a profile page that just sits there, don't expect any benefit for SEO. If you become positively active in the community there might be some value, but it's not something you should count on.
That said, one potential benefit from setting up social profiles that you don't actively use would be for protecting your brand name(s) against impostors.
Been reading some interesting blog content about FB.
One point is that it is much more about 'relating' to your customers rather than selling to them, if that makes sense. I guess it does, to some degree. In the long run, relating means the same thing as selling I would guess. But it is also less immediately effective in getting cash/sales of course.