Foursquare has updated Swarm app in Windows Phone Store to v1.2.1.18. This new update fixes bugs which caused issue while adding friends and signed in using Facebook login. 
Change log:
Notable fixes:
– Issues while adding friends
– Issue with facebook sign-in in some cases
Thanks for your patience as we iron out the last of the bugs that a few user are experiencing!
Download it here from Windows Phone Store for free.