I am a member of a number of sites and what I have found it is very hard to get them to give up some of their loyalty to the site they are used to.
If I approach a member of another site I say I am not asking you to leave your favourite site, just if you have some spare time check out our site.
When I first joined CHF I managed to bring in a number of new members, however now it seems much harder.
I think most sites are struggling to get new active members.Some join and you never see them again.