If you already have an account at Ancestry.com, make sure you add your ancestor's siblings onto your tree. Sometimes you can find a tree at Ancestry from the offspring of one of your ancestor's siblings that will bring you to *nearly* present day. But even if there isn't a tree to find, if you can trace an ancestor's sibling's name (and/or) their offspring forward to the 1930 US Census, you can generally find their present day family on Veromi.com, Privateeye.com, or Intellius.com. Double check to make sure that the general location of the present day person matches the information you already have on the sibling's offspring. Often times, especially with the more common names, you need to sort out which person with that name is the correct one.
These websites often include the names and locations of people that are surely dead by now. There aren't that many people over the age of 100 running around but I've used the names of my great-aunts and uncles over the age of 100 to find my cousins. For example, my grandfather Fred Pratt is still being listed on these websites - today is his 107th birthday. This is the same methodology I use when I'm volunteering my time to reconnect unclaimed people at various coroner's offices with their next of kin over at UnclaimedPersons.org. (We don't personally contact the next of kin, the coroner's office does that). You can also find your long lost friends/neighbors this way as well.
Once you have a present day/living person's name from your branch, try looking for them on facebook. I've found facebook friends/cousins Mike Towle, Steve Pratt, Nancy Birdsall, Bryan Baker and Melinda Callahan-Evans this way.