SIP was adopted by 3GPP in 1999 to be used in the mobile networks. Of course, that trickled throughout the telcos, so it now is widely used in the telco space. H.323 was widely used for PSTN backhauling to reduce tolls. However, SIP is more prevalent there, I'd guess.
Enterprise networks still use both H.323 and SIP, with more desk phones using SIP and videoconferencing equipment split, but moving to SIP.
That said, any move to SIP these days is only because there is nothing better. WebRTC is the next big thing. It borrows SDP from SIP, but no signaling protocol is defined.
Personally, I'd like to realize the vision behind what was intended to be H.325, which was to be a JSON-based signalling protocol that is fully distributed. Each application could implement whatever protocol it wanted, and they would all be associated through H.325, giving the user the feeling that these different applications work closely together. That would be possible, even when applications run on different devices.
Alas, there seems to be no financial motivator for those that control the market to explore that concept, so we're stuck with H.323, SIP, and WebRTC (and various proprietary glue pieces). The trend these days seems to be toward proprietary solutions, providing limited interop with others through SIP. Limited, as in nothing but basic voice and video.
