hey aelsharawi,
in general combining css files is a good thing as this reduces http requests. at the other hand you have to be careful by doing so as there are some known limitations - for example internet explorer only allows a certain amount of imports / rules etc. the easysocial main files are therefore split to not run into these named issues.

hope that helps and have a nice day

Yep, Alex is right and to be precise, I think IE limits 4095 rules per css file. If you have anything more than that IE will not render the rest of the css codes You can learn more about it here, http://blogs.msdn.com/b/ieinternals/archive/2011/05/14/10164546.aspx