We're happily chatting thru whatever site, and then all of a sudden they're like 'Now we'll start using whatsapp instead of this site my number is xxxxx xxx xxxx'.
And I I think, er, no, I'd rather keep chatting on this site. I don't necessarily WANT to give you my phone number or connect with you on WhatsApp - why not ASK me first instead of assuming?
What's so bad about chatting on the site anyway - you're messaging aren't you?!