I think there is a fairly normal 'settling in' period for new transplants. I came from the old Dave's Garden many years ago, and until I became familiar with this site, I double-dipped into both sites. Once I established friends on this site, my DG membership and any allegiance to that site just fizzled away.
I've never been too attracted to Houzz (it seems overwhelming to me) but have noticed a lot of Houzz rose people trying out this site. I presume there are other garden sites out there as well.
Rick, your suggestion is very welcoming to the new folks, but it strikes me as a bit 'neener-neener' to the sites they are leaving. I noted there is a thread within the rose forum where a lot of Houzz transplants are introducing themselves, identifying their old screen names, and just kind of catching up again. Perhaps that is enough.