My understanding is that you can have backlinks from ANY site and it shouldn't hurt you - if however you link out to bad neighbourhoods or irrelevant sites then you could get penalised.
so we suppose to give some love links and on the other hand, if we enable dofollow, that we bear the risk of getting bad neighbourhoods? if we have all these backlinks, and we nofollow. is that a good practice?
Are you kidding us ? Is it OK ? A link is a link is a LINK - grab it and say thanks to the unknown internet soldier who twitted your site's URL !
Since Topsy is linking back to our site, there is no harm of accepting its backlink. Also topsy is not a Spam. Its a tweet search engine.