My whole social circle are Meet Up friends.
Activity meet up groups are great... especially relating to nature and hiking. These are the two most popular groups in my city. The dance groups are also going strong... both lessons and meeting together at a club. The huge decline has been in general social groups... also dating groups are now dead (never were popular). Tons of singles have paired up doing specific activities ( hiking, etc) rather than 'dating'.
Bottom line... it's all about the organizer. How does he/she welcome newbies, keep people enthused, organize events, etc.