The best replacement for picasa is flickr.

The best alternatives to picasa are rated as following:
★★★★★  flickr
★★★★  iphoto
★★★  irfanview
★★  lightroom
★★  photoshop
★★  gimp
★★  shutterfly
★  photoshop elements
★  photobucket
★  google+
★  windows photo gallery
★  snapfish
★  acdsee
★  google+ photos
  flicker
  google photo
  aperture
  google drive
  picnik
  iphoto which is better
  iphoto 11
  photos
  google plus
  flickr better
  instagram
  google photos
  elements
  smugmug
  skydrive
  mobileme
  microsoft photo gallery
  lightbox
  windows live
  pictomio
  adobe photo album
  windows live photo gallery
  zoombrowser

Your alternative for picasa is not listed? Suggest!
Speaking about picasa, one probably shouldn't forget about iphoto.

Other search terms related to picasa and their ratings are:
★★★★★ iphoto
★★★ flickr
★ google+
★ scanning
picnik
web albums
hello
digital scrapbooking
You have in mind a companion for picasa and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of picasa in all years. Click on graph for more.
picasa
flickr
iphoto