That would depend on the rules for those sites. Most do not allow members to post links to other sites or personal contact details as that detracts/poaches the members from that site.
You would need to ensure you are not in breach of those sites T&C's. As a site, we (Socialswinging.com) cannot condone members from any site posting on other sites to attract new members.
If you can encourage others to join this site whilst not breaking any other sites rules, or by doing this by other means then that would be ok.