It’s the group messages that made me finally give in and join. Makes organising an after work pint much easier when everyone can see all the replies.
Sure, email does the same, as does a whole bunch of other messaging apps, but this was the one that hit critical mass amongst my friends. Having a client with basically no signup that works on pretty much everyone’s phone helped as well.
By comparison, enough people I know never went on facebook, that it never became the dominant way of organising meetups. Sure, I missed out on the odd party because it was only mentioned on the book of faces, but most of the time if someone was organising something they’d post it on face book, then text/email everyone as well.